PURPOSE: To facilitate measurement of sleeping state by a simple device, to improve an opening of power source by detecting a falling-asleep state, and to provide method and device to create simply a comfortabel sleeping environment.

CONSTITUTION: A head temperature of a person who is going to sleep is measured (S1), a body temperature of the person who has awaken, which is the head temperature of the person at a time when he wakes, is stored (S2), a past temperature a prescribed time ago, which is the head temperature of the person a second prescribed time before a current time, is stored (S3). When a difference between the body temperature at a time of awaking and the head temperature at a current time is equall to or larger than a first reference value of temperature difference, and also, a difference between the past temperature the prescribed time ago and the head temperature at the current time is equall to or larger than a second reference value of temperature difference, it is determined that a person has fallen asleep (S4), and it is determined that the person fell asleep a second set time before the time when it was determined that the person had fallen asleep.