Providing a thermal activation device for heat-sensitive self-adhesive sheet and a printer assembly employing the same, the device capable of thermally activating the heat-sensitive self-adhesive sheet in any of various patterns according the use thereof, and of developing at least a predetermined adhesive force on the overall surface of the sheet. The thermal activation device at least comprises a thermal head serving as thermally-activating heating means for thermally activating a heat-sensitive adhesive layer of the heat-sensitive self-adhesive sheet including a sheet-like substrate having a printable surface on one side thereof and the heat-sensitive adhesive layer on the other side thereof, and including an array of heat generating elements individually controllably energized; and energy control means for applying one or more shots of voltage pulse to the plural heat generating elements for energization thereby thermally activating an area of the heat-sensitive self-adhesive sheet that can be thermally activated by the thermal head in one step, and is characterized in that when plural shots of voltage pulses are applied to the heat generating elements of the thermal head for thermally activating the heat-sensitive self-adhesive sheet, the energy control means can selectively change a heat generating element(s) to be energized by the voltage pulse each time the voltage pulse is applied.