An inexpensive warm-up apparatus for an internal combustion engine having a simple structure and eliminating the need of forming a hole in a heat storage material retainer or the need for a seal around the hole or an electrical circuit. A heat storage material retainer (14) accommodating a latent heat type heat storage material (X) is arranged in a water jacket (13). The heat storage material retainer (14) has a nucleation device (2) that accelerates phase transition of the heat storage material in a supercooled state. The nucleation device includes a nucleation portion (24) for nucleating the heat storage material (X) in the supercooled state for phase transition, a hammer (25) supported in a movable manner, a target member (23) for applying impact to the nucleation portion by abutting against the nucleation portion when the hammer moves forward, and a lock mechanism (26) for restraining the hammer in the standby position by making use of the volume increase that occurs as the heat storage material undergoes phase transition to the liquid phase when receiving heat. The lock mechanism is released by vibrations generated when the internal combustion engine is started. The lock mechanism causes the hammer to abut against the target member to activate the nucleation device and nucleate the heat storage material.