A sleep depth determining device 2 is a device for determining an object person M's depth of sleep using a sensitivity threshold which is the minimum stimulus intensity that can be sensed by the object person M in a sleep state and includes: a stimulus giving unit 5 that gives a first stimulus P1 and a second stimulus P2 stronger than the first stimulus P1 to the object person M; a sense detecting unit 6 that detects whether the object person M senses the first stimulus P1 and the second stimulus P2; and a sleep depth determining unit 7 that determines that the object person M's depth of sleep is a depth of sleep S2 corresponding to a sensitivity threshold between a first stimulus intensity T2a and a second stimulus intensity T2b when the sense detecting unit 6 detects that the object person M does not sense the first stimulus P1 but senses the second stimulus P2. Accordingly, it is possible to precisely determine that the object person M's depth of sleep is the depth of sleep S2.