To allow an operator to select, at his or her discretion as necessary, between locking and unlocking an axle by providing means of selecting the function between an automatic control of locking axle by means of a lock command and a manual forced unlocking of the axle according to vehicle operating conditions.

When an auto mode is selected on a selector switch 14, a controller 10 issues a drive command to open or close a solenoid valve 3, automatically controlling locking or unlocking an axle according to vehicle operating conditions. This ensures stabilized turning and straight-ahead running regardless of a lift value and a side force. When a manual mode is selected on the selector switch 14, a close command voltage is forcibly applied to the solenoid valve 3, thus locking the axle. When a manual unlocking mode is selected, an open command voltage is applied forcibly to the solenoid valve 3, thus unlocking the axle. This allows an operator to make his or her own decision to ensure stabilized running even under conditions that would otherwise make running unstable if the automatic control was used.