An electric heater comprising: a heater assembly made up of a fin radiator, an electric heating element, and an electrode plate for supplying electric power to the heating element which are laid to overlap each other in a first direction, said heater assembly having first ends (301, 302) opposed to each other in the first direction and second ends opposed to each other in a second direction perpendicular to the first direction, said heater assembly also having a first and a second major surface which are opposed to each other in a thickness-wise direction thereof and through which a medium to be heated passes the fin radiators; first and second retainer frames (21 and 22); and metallic clamping members jointed to said first and second retainer frames (21 and 22), respectively, to produce a compression pressure acting on said first and second retainer frames (21 and 22) to clamp the fin radiator, the heating element, and the electrode plate together in the first direction, each of said metallic clamping members having a length made up of a support strip, a first clamping arm, and a second clamping arm extending from ends of the support strip, each of the support strips being placed on said heater assembly so that the first and second clamping arms (11 and 12) clamp said first and second retainer frames (21 and 22) to produce the compression pressure, characterized in that said first and second retainer frames (21 and 22) are placed on the first ends (301, 302) of said heater assembly, respectively, and are clamped together in the first direction, each of said first and second retainer frames (21 and 22) is made of a hollow strip member (21; 22) which has openings (213, 214; 223, 224) oriented to the second ends of said heater assembly, the first and second clamping arms (11 and 12) being fitted into the openings (213, 214; 223, 224) of said first and second retainer frames (21 and 22) and which are placed on the second ends of said heater assembly to serve as side frames, the first and second clamping arms (11 and 12) and/or said first and second retainer frames (21 and 22) have engagement portions which establish firm engagement between the first clamping arm and said first retainer frame and between the second clamping arm and said second retainer frame, the support strips are placed on the second ends of said heater assembly to serve as side frames, and wherein said metallic clamping members engage said heater assembly to define detent mechanisms which work to lock movement of said heater assembly in the thickness-wise direction thereof. a heater assembly made up of a fin radiator, an electric heating element, and an electrode plate for supplying electric power to the heating element which are laid to overlap each other in a first direction, said heater assembly having first ends (301, 302) opposed to each other in the first direction and second ends opposed to each other in a second direction perpendicular to the first direction, said heater assembly also having a first and a second major surface which are opposed to each other in a thickness-wise direction thereof and through which a medium to be heated passes the fin radiators; first and second retainer frames (21 and 22); and metallic clamping members jointed to said first and second retainer frames (21 and 22), respectively, to produce a compression pressure acting on said first and second retainer frames (21 and 22) to clamp the fin radiator, the heating element, and the electrode plate together in the first direction, each of said metallic clamping members having a length made up of a support strip, a first clamping arm, and a second clamping arm extending from ends of the support strip, each of the support strips being placed on said heater assembly so that the first and second clamping arms (11 and 12) clamp said first and second retainer frames (21 and 22) to produce the compression pressure, characterized in that said first and second retainer frames (21 and 22) are placed on the first ends (301, 302) of said heater assembly, respectively, and are clamped together in the first direction, each of said first and second retainer frames (21 and 22) is made of a hollow strip member (21; 22) which has openings (213, 214; 223, 224) oriented to the second ends of said heater assembly, the first and second clamping arms (11 and 12) being fitted into the openings (213, 214; 223, 224) of said first and second retainer frames (21 and 22) and which are placed on the second ends of said heater assembly to serve as side frames, the first and second clamping arms (11 and 12) and/or said first and second retainer frames (21 and 22) have engagement portions which establish firm engagement between the first clamping arm and said first retainer frame and between the second clamping arm and said second retainer frame, the support strips are placed on the second ends of said heater assembly to serve as side frames, and wherein said metallic clamping members engage said heater assembly to define detent mechanisms which work to lock movement of said heater assembly in the thickness-wise direction thereof.