To measure the sleep stage of a subject in on-invasive state by extraction a palmus signal and a breath signal from output signals of a living body signal detecting means disposed under the body of the subject, and discriminating a non-REM sleep and as REM sleep from at least one variation pattern between a palmus signal waveform and a breath signal waveform.

In a bed 7 where a subject lies, an air mat is disposed in a position to which the body of a tested person is applied, and a micro-differential pressure sensor 13 and an absolute pressure sensor 2 are installed through an air tube 12 connected to one end of the air mat 11 to constitute a living body signal detecting means 1 for detecting a living body signal of the subject. A signal extract means 4 of a sleep stage determination control device 3 is adapted to extract a signal by application of a suitable filter and a data processing means, and a sleep stage determining means 5 determines the sleep stage from a palmus signal and a breath signal extracted by the signal extract means 4. The determination result of the sleep stage is continuously recorded.