To provide a warm-up judging device and its method capable of shortening a time until the start of the operation of this device at the time of resetting the device.

This warm-up judging device for judging the warm-up state of an amplifier is provided with a first temperature sensor arranged near the amplifier, a second temperature sensor arranged farther away from the amplifier than the position where the first temperature sensor is arranged, a memory which stores information related with a temperature difference being the judgment reference of the validity/invalidity of warm-up, and a central processing unit which reads each signal from each temperature sensor, and calculates the temperature difference, and compares the temperature difference being the judgment reference with the calculated temperature difference, and when the calculated temperature difference is not less than the temperature difference being the judgment reference, instructs the start of the operation of the device.