Abstract: Provided is a constant monitoring system for a railway vehicle that is monitorable of a dimensional change of a railroad truck during running. The present disclosure is the constant monitoring system for the railway vehicle that includes a sensor and a calculator. The sensor detects passage therethrough of wheels included in the railroad truck of the railway vehicle. The calculator calculates an inter-axle distance between two axles of the railroad truck based on a detection result obtained from the sensor. The sensor includes two displacement sensors of non-contact type that detect the passage of the wheels from respective different locations.