A heat-sensitive adhesive layer is thermally activated to develop satisfactory adhesion at improved energy efficiency. A thermal activation thermal head is driven in sync with movement of a heat-sensitive adhesive sheet conveyed, and chosen heating elements stop being driven at a given timing. For instance, while moving the heat-sensitive adhesive sheet, driving of three heating elements (10B, 10F and 10J) and driving of two heating elements (10D and 10H) are alternately stopped whereas five heating elements (10A, 10C, 10E, 10G and 10I) are driven all the time. Supposing that the entire surface of a heat-sensitive adhesive layer is gridded to form a matrix, a region (15A) that is directly heated by none of the opposing heating elements 10 is placed regularly in a manner that makes its surrounding regions (15B to 15I) heated directly by the opposing heating elements. The directly heated regions (15B to 15I) are activated by the opposing heating elements (10) whereas the indirectly heated region (15A) is activated by heat transmitted from the surrounding regions (15B to 15I).