A heat sealer capable of providing a high sealing strength by appropriating a distribution of temperature at two sheet contact faces (19a, 19b) includes a block (31) made of a metallic material having a high rigidity and operable to heat the resinous sheet (S) while pressing the latter. The block (31) is provided with heaters (24a, 24b) therein and also with heat conducting members (33a, 33b) each having an excellent heat conductive characteristic higher than that of the metallic material. The heat conducting members (33a, 33b) are positioned between the heaters (24a, 24b) and the sheet contact faces (19a, 19b), respectively.