A thermal activation device includes a platen lock mechanism (19) for locking a platen roller (12) and a thermal activation head (11) into press contact with each other, the platen lock mechanism (19) including lock arms (32a and 32b) provided to be rotatable and engaged with a platen unit (18), and a compression coil spring (24) which urges the lock arms (32a and 32b) so as to bring the lock arms (32a and 32b) into engagement with the platen roller (18). The plate lock mechanism (19) includes lock pins (33) provided in the platen unit (18), and engagement grooves (37) which are provided in the lock arms (32a and 32b) and with which the lock pins (33) are engaged. Moreover, in the platen lock mechanism (19), in a state where the thermal activation head (11) is in press contact with the platen roller (12), an axis of the lock pins (33) is provided at a position farther than a position of a rotation center of the platen roller (12) with respect to a rotation center of the lock arms (32a and 32b).