before the description of the present invention proceeds , it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings . refering to fig5 to 8 , one embodiment of the present invention will be described in detail hereinafter . in fig5 a heat cooking apparatus according to the present invention includes , a pair of flat , planar heaters 8 provided outside the ceiling and base of the cubic heating chamber 1 having six walls perfectly contacting each other , respectively . each of said flat heaters 8 is fixed to the ceiling or base of the heating chamber 1 by attaching metal plates 9 . heat insulating materials 10a , b are provided outwardly on the metal plate 9 so as to prevent heat emission toward the outside of the heating chamber 1 . a magnetron 6 , provided on a waveguide 7 , is adapted to heat food 5 positioned on a pan 4 by microwave heating . with the above described construction , food 5 can be heated be either electrical heating or microwave heating . since the flat heater 8 has substantially the same area as the ceiling of the heating chamber 1 having a small convex curvature , the food 5 can be positioned extremely close to the ceiling , by the pan and this construction can more effectively use the space that is conventionally occupied by the sheathed heater . thus , the volume which can be effectively heated in a short time surrounding the pan 4 , between the ceiling of the heating chamber 1 and the four wise walls of the heating chamber 1 , is dramatically larger compared to the volume of a conventional apparatus . since the distance from food 5 placed on the pan 4 to the ceiling of the heating chamber 1 can be decreased , the temperature of the material will rapidly rise . consequently , food is rapidly cooked without losing its delicious qualities . the inner side of the ceiling of the heating chamber 1 is provided with a self - cleaning enamel layer 11 , which provides good heat emission . that is , the temperature of the food rises rapidly , resulting in a large quantity of oil and water stains on the self - cleaning layer , and , the self - cleaning enamel layer reaches a temperature best suited for the self - cleaning function , so that all oil stains are eliminated from the layer . however , such a characteristic is not found in a conventional apparatus . in fig6 and 7 , the flat heater 8 comprises a heating element 8a wound around a winding base 8b made of a heat resistant and insulating material such as mica . sandwiching this assembly are insulating plates 8c made of mica . by this construction , the flat heater assembly 8 has a property of flexibility in the perpendicular direction to the surface of heating chamber 1 to facilitate an easy fit to the ceiling of the heating chamber 1 . radially provided on an attaching plate 9a for the flat heater 8 are several slotted holes . a stepped screw 12 is inserted into the slotted hole to sandwich and tighten the flat heater 8 to the ceiling of the heating chamber 1 . in this apparatus , the ceiling of the heating chamber 1 has a gentle convex curvature toward the flat heater 8 . with the above described construction , if and when the flat heaters 8 and attaching plate 9a deform by thermal expansion with the temperature rise in the apparatus , the attaching plate 9a can expand in a planar direction because the stepped screw 12 is loosely received in the slotted hole . however , the stress caused by heat expansion in the ceiling of the heating chamber 1 works in a perpendicular direction to the bend of the ceiling upwardly because the ceiling of the heating chamber 1 is restricted at its four sides . accordingly , the contact between the flat heater 8 and the ceiling of the heating chamber 1 is tightened by this heat expansion . in addition , the flat heater 8 provided at the base of the heating chamber 1 is attached with a metal plate 9b for the same purpose . in this case , however , the metal plate 9b is fitted to the base with a bar shaped , flexible band 13 illustrated in fig5 in order to facilitate an easy assembly operation . furthermore , provided on the attaching plate for the upper flat heater 8a are cutouts extending along diagonal lines from the center toward the outer corners . with the above described construction , if and when the flat heater 8 and attaching plate 9a deform by thermal expansion with the temperature rise in the apparatus , the attaching plate for the upper flat heater 9a is expanded in the same manner as mentioned above to push up to contact the ceiling of the heating chamber 1 closely reducing the clearance of said cutouts . as illustrated in fig8 ( a ) and 8 ( b ) the heating chamber ceiling 14 is constituted to have a gentle convex curvature toward the outside of the heating chamber . when the heating chamber ceiling expands by heat from the heater ( not shown in the figure ), the ceiling deforms as shown in fig8 ( a ) because the four sides of the ceiling are fixed and it cannot expand toward the walls of the heating chamber . in this case , forces shown in fig8 ( a ) are applied to the point p on the ceiling , that is , a compression force f1 is applied to the inner face , and a tensile force fθ is applied to the outer face of the ceiling 14 having a thickness t . on the contrary , if the ceiling has a curvature extending toward the inside of the heating chamber as shown in fig8 ( b ), tensile force fθ is applied to the inner face of the heating chamber with the enamel layer , and compression force f1 is applied to the outer face of the heating chamber . an apparatus according to the present invention is provided with a self - cleaning enamel layer 11 at the ceiling of the heating chamber . the enamel layer is mainly composed of glass and inorganic materials and the layer resists compression force , but is extremely weak against tensile force . it has been clearly demonstrated by experimental trial that these characteristics are especially true of a self - cleaning layer equal to or thicker than 300 micron meters . that is , continuous and intermittent blank baking tests were carried out , simulating grill cooking , using an apparatus with the heating chamber ceiling 14 made from an aluminized steel sheet formed an aluminum porcelain enamelling layer . as a result , all heating chamber ceilings 14 shaped as shown in fig8 ( b ) having thicknesses of 1 ; 0 . 6 , 2 ; 0 . 8 and 3 ; 1 . 0 mm cracked on their enamel layers within 80 - 120 hours . on the contrary , the heating chamber ceiling 14 shaped as shown in fig8 ( a ) has never cracked on its enamel layer after 500 hours . accordingly , the theory on which the present invention is based has been proved to be true by these experimental trials . as is clear from the preceding description , the following advantages can be attained by the heat cooking apparatus according to the present invention . ( 1 ) the heat cooking apparatus according to the present invention is provided with electrical heaters in a flat shape . when the flat heater is provided above the heating chamber , a self - cleaning layer on the heating chamber ceiling inside reaches 300 °- 450 ° c . where the catalytic action of the self - cleaning layer works effectively , during the actual cooking operation . moreover , by adopting a flat heater , food positioned in the heating chamber can be heated more uniformly . accordingly , even when food to be heated is positioned nearer to the heating chamber ceiling heat source , the ceiling which is hardly cleaned in a conventional apparatus can be kept clean , and the effective capacity of the heating chamber is enlarged . this is a main characteristics of the flat heater of the present invention , and in this construction , there is no protrusion in the heating chamber which can be kept clean both actually and visually . ( 2 ) by designing the ceiling of heating chamber to have a convex curvature toward the flat heater , the ceiling can contact the flat heater perfectly during heating so that heat from the flat heater can be effectively transmitted toward the heating chamber to increase total heat efficiency . ( 3 ) local and uneven heating characteristics can be eliminated to improve the cooking quality . ( 4 ) local overheating of the heater can be eliminated which reduces wire damage or insulation degradation in heating elements . ( 5 ) the heater can be disposed outside the heating chamber with high heating efficiency , and thus , there is no protrusion in the heating chamber , thereby facilitating cleaning and handling of the heat cooking apparatus . ( 6 ) the walls of the heating chamber always deform in one direction during heating , thus permitting an apparatus design with stable and uniform heat distribution during microwave heating , and reduction in uneven heating of food by microwave heating . ( 7 ) a keep plate for the flat heater is slidably attached to the heating chamber to eliminate the wave shape deformation on the outer periphery so that the flat heater can contact perfectly with the ceiling of the heating chamber at any heating stage to effectively transmit heat from the flat heater toward food material , increasing the total heat efficiency . ( 8 ) a self - cleaning function layer such as a self - cleaning enamel layer provided on the ceiling of heating chamber is black or dark grey so that it works as a so - called blackbody to have superior heat absorption and heat emission characteristics . accordingly , the layer can absorb and transmit high temperature heat from the flat mica heater , which has superior insulation ability , toward food material positioned in the heating chamber . ( 9 ) the ceiling of the heating chamber provided with a self - cleaning layer has a convex curvature toward the outside . when the ceiling expands due to heat from the electrical heater or microwave , a compression stress is mainly aplied to said self - cleaning layer to prevent it from cracking or flaking , so that the temperature of the self - cleaning layer can be increased up to the maximum allowable point . accordingly , by displaying a self - cleaning layer on the upper part of the heating chamber , a heating chamber with an efficient grill cooking function , stable self - cleaning ability , and long service life can be manufactured . therefore , according to the present invention , an electric heating apparatus is provided that is simple in construction , is highly reliable , durable , and safe , has uniform and rapid heating capabilities , enlarged effective capacity , and facilitates cleaning of the heating chamber , wherein those characteristics are the superior features of a flat heater . although the present invention has fully been described in connection with the preferred embodiment thereof , it is to be noted that various changes and modifications are apparent to those skilled in the art . accordingly , such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims , unless they depart therefrom .