To provide a parking space monitoring system, a monitoring device and an in-vehicle device for preferentially guiding a vehicle to an unoccupied space having high imaging precision by a monitoring camera or an in-vehicle camera.

The parking space monitoring system includes: the in-vehicle device which controls the in-vehicle camera; the monitoring camera which images a parking space; a vehicle detection device which detects a vehicle; and the monitoring device connected to the devices for monitoring the parking space. The in-vehicle device transmits second monitoring information indicating a range that can be imaged by the in-vehicle camera to the monitoring device. The monitoring device includes: a recording part recording first monitoring information indicating a range that can be imaged by the monitoring camera; and an unoccupied space notification part which determines an unoccupied space that can be imaged by the monitoring camera or the in-vehicle camera based on the first monitoring information or the second monitoring information, and transmits unoccupied space information indicating the determined unoccupied space to the in-vehicle device. The in-vehicle device creates an image indicating the received parking space information or the unoccupied space information, and displays the image to a display device.