To provide a monitor allowing further reduction of power consumption and heat generation of the monitor itself.

In this monitor 11, an internal temperature measurement part 24 measures an internal temperature of the monitor 11. A control part 25 controls transition of a state such that the state transits from a first state to a second state when it is decided that the internal temperature is a first threshold value or above in the first state and that the second state is maintained for a prescribed period, that the second state is further maintained for a prescribed period when it is decided that the internal temperature is not less than a second threshold value smaller than the first threshold value when the prescribed period lapses, and that the state transits from the second state to the first state when it is decided that the internal temperature is less than the second threshold value. The monitor 11 can be applied to a monitor monitoring a liquid level of a liquid stored inside a tank.