referring to fig1 of the drawings , a heat exchanger according to this invention comprises a first header 1 and a second header 7 mounted on the top of a heat exchanging portion 6 , a first fluid supply member 3 mounted on a lateral face of the first header 1 and a second fluid supply member 9 mounted on the lateral face of the second header 7 . this heat exchanger is used as a vaporizer which vaporizes methanol and water in a fuel cell system by heat exchange with a high temperature gas . the first header 1 comprises a top plate 1 a , a bottom plate 1 b and a comb - shaped plate 2 . referring to fig2 the top plate 1 a and the bottom plate 1 b are fixed to each other by soldering or welding with the comb - shaped plate 2 vertically sandwiched therebetween . referring again to fig1 the comb - shaped plate 2 comprises a plurality of arms 2 b disposed in parallel at equal intervals and a connecting member 2 a connecting an end of each arm 2 b . the first fluid supply member 3 seals the opening on the lateral face of the header 1 which is positioned to face another end of each arm 2 b . the length of the arms 2 b is set so that a fluid distribution space sb is formed between the first fluid supply member 3 and the end of the arms 2 b . the thickness of the comb - shaped plate 2 is approximately one millimeter . the space between adjacent arms 2 b forms a conducting passage 5 which extends at right angles to the cross - section of the header 1 shown fig2 . the upper end of the conducting passage 5 is delimited by the top plate 1 a and the bottom end is delimited by the bottom plate 1 b . each conducting passage 5 is connected to the fluid distribution space sb . it is preferred that the width of the conducting passage 5 is set to the minimum value which satisfies a liquid methanol supply amount from the header 1 to the heat exchanging portion 6 . the width of the arms 2 b is determined based on this condition . two fluid outlets 3 a which open towards the fluid distribution space sb are formed in the first fluid supply member 3 . the fluid outlets 3 a are connected to a methanol supply pump via two supply pipes 3 b which project from the fluid supply member 3 in an opposite direction to the fluid outlet 3 a . through holes 4 are formed at equal intervals on the bottom plate 1 b at positions facing each conducting passage 5 . the second header 7 comprises a top plate 7 a , a bottom plate 7 b and a comb - shaped plate 8 . the comb - shaped plate 8 comprises a plurality of parallel arms 8 b and a connecting member 8 a . the fluid supply passage 9 is provided with only a single fluid outlet 9 a which is connected to a water supply pump via a connecting pipe 9 b . the basic structure of the second header 7 is the same as the first header 1 . a fluid distribution space sc is formed between the arms 8 b and the second fluid supply member 9 and a conducting passage 50 is formed between adjacent arms 8 b . through holes 40 are formed on the bottom plate 7 b at equal intervals at positions facing each conducting passage 50 . the first header 1 and the second header 7 are disposed adjacent to each other on the top of the heat exchanger 6 . however the two headers 1 and 7 are disposed so that the direction of flow in the conducting passage 50 of the second header 7 and the direction of supply of fluid from the second fluid supply member 9 to the second header 7 are respectively orthogonal to the direction of flow in the conducting passage 5 of the first header 1 and the direction of supply of fluid from the second fluid supply member 3 to the first header 1 . the heat exchanging portion 6 is formed by alternately laminating a high temperature fluid layer 61 which allows flow of high temperature fluid and a low temperature fluid layer 62 which allows flow of a low temperature fluid in a vertical direction . these layers are laminated perpendicular to one another . the high temperature layer 61 and the low temperature layer 62 are insulated by a partition . a plurality of passages 61 a which pass horizontally through the layers are formed in the high temperature layers 61 . a plurality of passages 62 a which pass vertically through the layers are formed in the low temperature layers 62 . each passage 61 a has an opening on a lateral face of the high temperature fluid layer 62 . high temperature gas is supplied in the direction of an arrow in fig1 in each passage 61 a . the high temperature gas passes horizontally through each passage 61 a and is discharged from the opposite end of the heat exchanging portion 6 . each passage 62 a has an opening on the top end of the low temperature fluid layer 62 . the through hole 4 in the bottom plate 1 b of the first header 1 and the through hole 40 in the bottom plate 7 b of the second header 7 are disposed so that both are directly above the passage 62 a . in this heat exchanger , liquid methanol is supplied to the first header 1 through the first fluid supply member 3 and water is supplied to the second header 9 through the second fluid supply member 9 . the second header 7 is disposed upstream of the first header 1 with respect to the flow of high temperature gas in the passage 61 a . the boiling point of the water supplied to the second header 7 is higher than the boiling point of methanol supplied to the first header 1 . in order to effectively vaporize two different kinds of liquids , a header handling liquid with a higher boiling point should therefore be disposed at a position nearer to the inlet of the passage 61 a than a header handling a liquid with a lower boiling point . in the first header 1 , liquid methanol is supplied to each conducting passage 5 from the fluid distribution space sb and drips in equal amounts from the through holes 4 to the passage 62 a of the low temperature fluid layer 62 . however , of the passages 62 a in the low temperature fluid layer 62 , liquid methanol only drips into the passages 62 a positioned below the first header 1 . liquid methanol does not drip into the passages 62 a positioned below the second header 7 . in the second header 7 , water is supplied to each conducting passage 8 b from the fluid distribution space sc and drips from the through hole 40 down into the passage 62 a of the low temperature fluid layer 62 . however , of the passages 62 a in the low temperature fluid layer 62 , water only drips into the passages 62 a positioned below the second header 7 . water does not drip into the passages 62 a positioned below the first header 1 . thus the liquid methanol and water dripping down into the low temperature fluid layer 62 do not mix and are respectively heated and vaporized by heat exchange with the high temperature gas of the high temperature fluid layer 61 . the methanol vapor and water vapor are separately discharged from the lower end of the low temperature fluid layer 62 . the fuel cell system may mix discharged fuel vapor and water vapor before supplying them to a reformer , or alternatively , they may separately be supplied to the reformer . thus water and methanol are not mixed at all in the heat exchanger and are vaporized in separate passages . therefore when varying the mixing ratio of methanol vapor and water vapor , varying the supply amount of water or liquid methanol to the heat exchanger makes it possible to vary the mixing ratio of methanol vapor and water vapor discharged from the heat exchanger with high response characteristics . in this embodiment , the high temperature gas corresponds to a high temperature fluid , methanol corresponds to a first low temperature fluid and water corresponds to a second low temperature fluid . the top plate 1 a corresponds to a first top plate and the top plate 7 a corresponds to the second top plate . the bottom plate 1 b corresponds to a first bottom plate and the bottom plate 7 b corresponds to a second bottom plate . the passage 62 a positioned below the first header 1 corresponds to a first fluid passage or alcohol passage and the passage 62 a positioned below the second header 7 corresponds to a second fluid passage or water passage . the passage 61 a corresponds to a high temperature fluid passage . the first header 1 and the first fluid supply member 3 correspond to a first supply mechanism or an alcohol supply mechanism . the second header 7 and the second fluid supply member 9 correspond to a second supply mechanism or a water supply mechanism . the holes 4 correspond to a first group of holes and the holes 40 correspond to a second group of holes . the fluid distribution space sb corresponds to a first fluid distribution space and the fluid distribution space sc corresponds to a second fluid distribution space . the conducting passage 5 corresponds to a first conducting passage and the conducting passage 50 corresponds to a second conducting passage . a second embodiment of this invention will now be described with reference to fig3 and 4 . in this embodiment , a single header 21 is used instead of the first header 1 and the second header 7 . a water passage and a liquid methanol passage are separately formed in the header 21 . referring to fig3 the header 21 comprises a top plate 21 a , a bottom plate 21 b and a comb - shaped plate 22 . the top plate 21 a and the bottom plate 21 b are fixed to each other by welding or soldering with the comb - shaped plate 22 vertically sandwiched therebetween . a fluid supply member 23 is mounted on the lateral face of the header 21 . three fluid outlets 23 a - 23 c are formed in the fluid supply member 23 and open towards the interior of the header 21 . the fluid outlets 23 a and 23 c are disposed on both sides of the fluid outlet 23 b and are respectively connected to a methanol supply pump via two connecting pipes 23 d , 23 f which project from the fluid supply member 23 in an opposite direction to the fluid outlets 23 a , 23 c . the central fluid outlet 23 b is connected to a water supply pump via a supply pipe 23 e which projects from the fluid supply member 23 in a direction opposite to the fluid outlet 23 b . referring to fig4 the comb - shaped plate 22 is provided with a long arm 22 b and a short arm 22 c which are disposed alternately at equal intervals . an end of the respective arms 22 b and 22 c are connected to a connecting member 22 a . another end of the long arm 22 b reaches the fluid supply member 23 and in this manner partitions the fluid distribution spaces sb 1 - sb 3 which independently face the fluid outlets 23 a - 23 c . a conducting passage 25 a ( 25 b , 25 c ) is formed between adjacent arms 22 b and 22 c . the conducting passages 25 a are connected to the fluid distribution spaces sb 1 , the conducting passages 25 b are connected to the fluid distribution spaces sb 2 , and the conducting passages 25 c are connected to the fluid distribution spaces sb 3 . referring again to fig3 holes 41 are formed at equal intervals in the bottom plate 21 b corresponding to each conducting passage 25 a - 25 c in a manner similar to the holes 4 formed in the bottom plate 1 b of the first embodiment . the structure of the heat exchanger 6 is the same as that described with reference to the first embodiment . in this heat exchanger , liquid methanol is only supplied to the conducting passage 25 a connected to the fluid distribution space sb 1 and the conducting passage 25 c which is connected to the fluid distribution space sb 2 . thus liquid methanol only drips into the passages 62 a of the low temperature fluid layer 62 which are positioned below the conducting passages 25 a and 25 c . on the other hand , only water is supplied to the conducting passage 25 c which is connected to the fluid distribution space sb 2 . thus water only drips into the passages 62 a of the low temperature fluid layer 62 which are placed below the conducting passage 25 b . high temperature gas is supplied to the high temperature fluid layer 61 of the heat exchanging portion 6 in the same manner as the first embodiment . at this time , the high temperature fluid layers 61 located at the center of the heat exchanging portion 6 tend to have a higher temperature than the high temperature fluid layers 61 located near to both lateral faces of the heat exchanging portion 6 . according to this embodiment , water which has a higher boiling point drips into the high temperature fluid layers 61 located at the center of the heat exchanging portion 6 . liquid methanol which has a lower boiling point drips into the high temperature fluid layers 61 located near to both lateral faces of the heat exchanging portion 6 . thus it is possible to perform highly efficient vaporization of the liquid methanol and water in the same manner as the first embodiment . the heat exchanger according to this embodiment makes it possible to process two types of fluid separately using a single header 21 . this allows the structure to be simplified and manufacturing costs to be reduced in comparison to the heat exchanger according to the first embodiment . in each of the embodiments above , fluid is supplied from the top of the heat exchanging portion 6 . however it is possible to mount a structural member equivalent to the first header 1 and the second header 7 or the header 21 on the lower face of the heat exchanging portion 6 and to discharge fluid vapor to the top of the heat exchanging portion 6 . furthermore it is possible to form the passages 61 a of the high temperature fluid layer 61 vertically , to form the passages 62 a of the low temperature fluid layer 62 horizontally and to mount a structural member equivalent to the header on the lateral face of the heat exchanging portion 6 . in each of the above embodiments , the fluid distribution spaces sb , sc , sb 1 , sb 2 , sb 3 and the conducting passages 5 , 50 , 25 a - 25 c are partitioned by the comb - shaped plates 2 , 8 , 22 inside the header 1 , 7 , or 21 . however it is possible to partition a plurality of spaces having a waveform cross section by press forming any of the top plates 1 a , 7 a , 21 a or the bottom plates 1 b , 7 b , 21 b . furthermore it is possible to assign the fluid distribution spaces sb , sc , sb 1 , sb 2 , sb 3 and the conducting passages 5 , 50 , 25 a - 25 c to these spaces . in this manner , the comb - shaped plate becomes unnecessary which reduces the cost of manufacturing the header and improves the rigidity thereof . in this embodiment , the fluid distribution spaces sb 1 and sb 2 correspond to a first fluid distribution space and the fluid distribution space sb 2 corresponds to the second fluid distribution space . the conducting passages 25 a and 25 c correspond to a first conducting passage and the conducting passage 25 b corresponds to a second conducting passage . the fluid outlets 23 a and 23 c correspond to a first outlet and the fluid outlet 23 b corresponds to a second outlet . the contents of tokugan 2001 - 60089 with a filing date of mar . 5 , 2001 in japan , are hereby by reference . although the invention has been described above by reference to certain embodiments of the invention , the invention is not limited to the embodiments described above . modifications and variations of the embodiments described above will occur to those skilled in the art , in light of the above teachings . the embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows :