A parking space monitoring device that calculates a parking space with higher accuracy. The parking space monitoring device includes an obstacle detection unit (2, 3, 4) that uses a relative distance (Ri) and relative angle (θi) between an obstacle (Cp) near a vehicle (1) and the vehicle to detect a relative position (Pi(R1, θi)) of the obstacle with respect to the vehicle, a memory (8) that stores the relative position (Pi) detected by the obstacle detection unit in association with movement information of the vehicle, and a parking space calculation unit (7) that calculates a parking space based on the relative information (Pi) and the movement information that are stored in the memory.