according to the present embodiment , the disclosed pump is applied to a two - valve piezoelectric pump 20 . as illustrated in fig1 to 3 , the piezoelectric pump 20 includes a lower housing 21 and an upper housing 22 in sequence from the bottom . the lower housing 21 has an inlet channel hole 24 and an outlet channel hole 25 for allowing cooling water ( liquid ) to pass therethrough . the inlet channel hole 24 and the outlet channel hole 25 are substantially perpendicular to the through - thickness plane of the lower housing 21 and are preferably substantially parallel to each other . a piezoelectric vibrator ( diaphragm ) 28 is supported between the lower housing 21 and the upper housing 22 via an o ring 29 so as to be sealed against leakage of liquid . a pump chamber p is formed between the piezoelectric vibrator 28 and the lower housing 21 . an air chamber a is formed between the piezoelectric vibrator 28 and the upper housing 22 . according to the present embodiment , the axis of the inlet channel hole 24 and that of the outlet channel hole 25 are preferably substantially perpendicular to the piezoelectric vibrator 28 . the piezoelectric vibrator 28 may be of a unimorph type and may have a central shim 28 a and a piezoelectric element 28 b laminated on one of both sides of the shim 28 a ( in fig2 , on the upper surface ). the shim 28 a is made of a conductive metal sheet , for example , a metal sheet having a thickness of approximately about 50 μm to about 300 μm made of stainless steel , 42 alloy , or other materials . the piezoelectric element 28 b is made of lead zirconate titanate ( pzt ) ( pb ( zr , ti ) o 3 ) and has a thickness of the order of about 300 μm . the piezoelectric element 28 b is poled in the direction of both sides thereof . such a piezoelectric vibrator is well known . the inlet channel hole 24 and the outlet channel hole 25 of the lower housing 21 are provided with ( umbrella ) check valves 32 and 33 , respectively . the check valve 32 is disposed adjacent to the pump chamber p and is an inlet check valve for permitting a fluid to flow from the inlet channel hole 24 to the pump chamber p and preventing reverse flow of the fluid . the check valve 33 is disposed adjacent to the outlet channel and is an outlet check valve for permitting a fluid to flow from the pump chamber p to the outlet channel hole 25 and preventing reverse flow of the fluid . the check valves 32 and 33 have the same form . the check valves 32 and 33 have perforated substrates 32 a and 33 a and elastic umbrellas 32 b and 33 b attached thereto , respectively . such umbrella check valves are well known . in the present embodiment , the perforated substrates 32 a and 33 a are separated from the lower housing 21 . however , they may be integrally molded with the lower housing 21 . according to the present embodiment , the lower housing 21 has the inlet channel hole 24 and the outlet channel hole 25 , as described above . in addition , the inlet channel hole 24 and the outlet channel hole 25 are connected to an inlet channel 26 and an outlet channel 27 , respectively . both the inlet channel 26 and the outlet channel 27 are formed by a first channel plate 40 and a second channel plate 50 . the lower housing 21 includes annular grooves 24 a and 25 a whose outer surfaces are opened ( see fig2 ). the annular grooves 24 a and 25 a are concentric with the inlet channel hole 24 and the outlet channel hole 25 , respectively . the annular grooves 24 a and 25 a may be eccentric as needed . the first channel plate 40 serves as both a first channel plate for the inlet side and that for the outlet side and is formed as a plate in which both the first channel plates are integrated with each other . the first channel plate 40 includes an inlet tubular protrusion 41 to be fit in the annular groove 24 a , an outlet tubular protrusion 42 to be fit in the annular groove 25 a , an inlet channel recess 43 communicating with the inlet tubular protrusion 41 , an outlet channel recess 44 communicating with the outlet tubular protrusion 42 , and a partition 45 separating the inlet channel recess 43 and the outlet channel recess 44 . the second channel plate 50 serves as both a second channel plate for the inlet side and that for the outlet side and is formed as a plate in which both the second channel plates are integral with each other , as in the case of the first channel plate 40 . the second channel plate 50 includes an inlet channel recess 53 ( which corresponds to the inlet channel recess 43 in the first channel plate 40 ), an outlet channel recess 54 ( which corresponds to the outlet channel recess 44 ), and a partition 55 ( which corresponds to the partition 45 ). the first channel plate 40 and the second channel plate 50 are joined together by joining of a joint surface 46 including the partition 45 and a joint surface 56 including the partition 55 by , for example , brazing using solder , thereby forming the inlet channel 26 extending to the inlet tubular protrusion 41 and the outlet channel 27 extending to the outlet tubular protrusion 42 . each of the first channel plate 40 and the second channel plate 50 is constructed of a sheet made of , for example , aluminum , copper , or stainless steel and having a thickness of approximately 0 . 1 mm to 0 . 3 mm , and the thickness of the channel is equal to or smaller than 1 mm . therefore , the thickness of the entire pump can be reduced . the inlet channel 26 and the outlet channel 27 are noncircular in cross section . the inlet channel 26 has a flat shape that has a greater width in a direction substantially perpendicular to the protruding direction of the inlet tubular protrusion 41 . similarly , the outlet channel 27 has flat shape that has a greater width in a direction substantially perpendicular to the protruding direction of the outlet tubular protrusion 42 . one of the outlet channel recesses 44 and 54 may be omitted such that only one of the first channel plate 40 and the second channel plate 50 has an outlet channel recess . the inlet tubular protrusion 41 is fit into the annular groove 24 a such that an o ring 47 is disposed between the outer surface of the inlet tubular protrusion 41 and the inner surface of the annular groove 24 a . similarly , the outlet tubular protrusion 42 is fit into the annular groove 25 a such that an o ring 57 is disposed between the outer surfaces of the outlet tubular protrusion 42 and the inner surface of the annular groove 25 a . this maintains sealing to prevent leakage of liquid . the inlet tubular protrusion 41 and the outlet tubular protrusion 42 include internal flanges 41 a and 42 a at the respective ends such that the inlet tubular protrusion 41 and the outlet tubular protrusion 42 are mechanically strengthened to avoid both protrusions from becoming flattened easily . in the present embodiment , the internal flanges 41 a and 42 a project toward the inside of the end of the inlet tubular protrusion 41 and that of the outlet tubular protrusion 42 , respectively . however , the internal flanges 41 a and 42 a may outwardly project . in this case , the o rings 47 and 57 are prevented from falling when the lower housing 21 and the first channel plate 40 are attached or detached from each other . for the piezoelectric pump 20 having the above described structure , when the piezoelectric vibrator 28 is elastically deformed ( vibrated ) in forward and reverse directions , on a stroke for increasing the volume of the pump chamber p , because the inlet check valve 32 is opened and the outlet check valve 33 is closed , liquid flows from the inlet channel 26 and the inlet channel hole 24 into the pump chamber p ; on a stroke for reducing the volume of the pump chamber p , because the outlet check valve 33 is opened and the inlet check valve 32 is closed , the liquid flows from the pump chamber p to the outlet channel hole 25 and the outlet channel 27 . therefore , a pumping action is produced by continuously causing the piezoelectric vibrator 28 to be elastically deformed ( vibrated ) in forward and reverse directions . in the present embodiment , both the channel from the inlet channel 26 to the pump chamber p and the channel from the pump chamber p to the outlet channel 27 are formed by the first channel plate 40 and the second channel plate 50 , and the first channel plate 40 includes the inlet tubular protrusion 41 and the outlet tubular protrusion 42 , which are fit in the annular grooves 24 a and 25 a being concentric with the inlet channel hole 24 and the outlet channel hole 25 , respectively . as a result , the size and the thickness of the entire system can be reduced . that is , the lower housing 21 and the upper housing 22 do not have a protrusion for connecting the pump chamber p to the inlet channel 26 and the outlet channel 27 , and a flexible tube also is not present . in the embodiment described above , the first channel plate 40 and the second channel plate 50 form both the inlet and outlet channels . however , the first channel plate and the second channel plate can be provided for each of the inlet channels and the outlet channels , i . e ., the first channel plate 40 and the second channel plate 50 can be separated by the partition 45 and the partition 55 , respectively , into different portions . in this case , the arrangement of the inlet channel hole 24 and the outlet channel hole 25 and the orientations thereof can be designed more flexibly . in the embodiment described above , the unimorph piezoelectric vibrator 28 is illustrated as a diaphragm . however , a bimorph piezoelectric vibrator can also be used . in the embodiment described above , the present invention is applied to a two - valve diaphragm pump in which the pump chamber p is disposed on only one side of the piezoelectric vibrator 28 . however , the present invention is applicable to a diaphragm pump in which a pump chamber is disposed on each of both sides of a piezoelectric vibrator . in addition , the present invention is applicable to general diaphragm pumps , which produce a pumping action by causing the volume of a pump chamber to increase and decrease in cycles .