A measurement unit (101) is attached to a measurement target person and measures an acceleration. A body motion calculation unit (104) obtains the magnitude of a body motion of the measurement target person based on the acceleration measured by the measurement unit (101). An activity state determination unit (105) time-serially obtains an activity state representing whether the measurement target person is in a first state (rising state) or a second state (lying state), based on a posture decided by a posture decision unit (103) and the magnitude of the body motion calculated by the body motion calculation unit (104). If the activity state after the activity state has transitioned continues for a predetermined time defined in advance, an activity state correction unit (106) determines that the transition of the activity state obtained by the activity state determination unit (105) has been done. A time correction unit (107) returns the transition time of the activity state determined by the activity state correction unit (106) by the predetermined time.
