PURPOSE: To heat the specimen in a specimen receiving container to constant temp. with high efficiency, by providing the transparent conductive film mounted to the transparent specimen housing container and a means for supplying a current to the conductive film to hold said film to a constant temp.

CONSTITUTION: When a perforated container 13 being a transparent specimen housing container is mounted to a transparent heater main body part 11 having a transparent conductive film 16 arranged thereto, a current is supplied to the conductive film 16 in such a state that the transparent conductive film 16 is mounted to the transparent part of the perforated container 13 through the transparent heater main body 11 to hold the conductive film 16 to a constant temp. By this method, a specimen is heated to a constant temp. with high heat efficiency by the efficient transfer of heat from the conductive film 16 to the perforated container 13 being the specimen housing container. Further, the specimen can be observed from a short distance in a state held to a constant temp. through the transparent conductive film 16 and the transparent part of the perforated container 13.