A warm-up apparatus for a vehicle including a system which charges a battery by using an external power supply is provided. The warm-up apparatus includes a heater for warming up, a current detector, a voltage detector and a resistance changer. The heater is mounted in the vehicle and produces heat by receiving electric power from the external power supply. The current detector detects an allowable current value of the external power supply. The voltage detector detects an output voltage value of the external power supply. The resistance changer changes an electric resistance value of the heater based on the allowable current value detected by the current detector and the output voltage value detected by the voltage detector.