To obtain a heat-sensitive adhesive sheet usable as a double post card capable of transmitting the same amount of information as that of a letter, excellent in stickiness of postage stamp and signature stamping on the released surface after removal, and provide an information recording unit using the sheet.

This heat-sensitive adhesive sheet is obtained by forming heat sensitive adhesive layers 4a and 4b on both sides of a plastic sheet comprising a plastic sheet substrate 1 having one matted side, and a release layer 3 which is laminated to the matted surface, can be released from a matte layer 2 and has excellent stickiness of postage stamp and signature stamping on the released surface after removal. The release layer 3 consists essentially of at least one resin selected from a polyvinyl alcohol, etc., and an inorganic pigment and/or an organic pigment in the weight ratio of the whole resin to the whole pigment in the layer of 100 (resin):0 (pigment) to 60:40. A heat sensitive adhesive sheet 5 is laid between paper to be folded into two or three parts having information recorded inside or two or more pieces of paper in which information is recorded and totally integrated by heating to give the objective information recording unit.