To easily grasp an elapsed period of time since the operation completion point of an electronic device without requiring the feeding of power, and also, without adding parts cost.

In a color heat-sensitive printer 2 as the electronic device, a relationship of the elapsed period of time t since the operation completion point to the time differential value dΘ/dt of a detected temperature Θ by a temperature detector 20 is stored in a ROM 33 in advance. When a power source is turned on under the control by a system controller 30, the time differential value dΘ/dt is calculated from the detected temperature Θ by a first calculation circuit 38. The elapsed period of time t corresponding to the time differential value dΘ/dt calculated by the first calculation circuit 38 is calculated by a second calculation circuit 39 by referring to the relationship of the elapsed period of time t to the time differential value dΘ/dt stored in the ROM 33.