A portable stop determining device 10 includes an acceleration sensor 115 and a state grasp unit 143 in order to detect that a vehicle such as a train stops, with an adequate degree of accuracy, even when position information of a user and time information are not available. The state grasp unit 143 outputs an alarm signal when detecting deceleration in a direction of travel of the vehicle based on a measured value from the acceleration sensor, and detecting that posture of the portable stop determining device carried by a person riding on the vehicle inclines in the direction of travel and then returns to a basic state.