An interlock circuit for a working vehicle having a vehicle body, a working implement attached to the vehicle body, a clutch mechanism for connecting and disconnecting power to the working implement, and a shifting device for determining a traveling direction of the vehicle body. The interlock circuit has a check function to check a backward operational run in which the vehicle body travels backward with the clutch mechanism engaged, and a release function to permit a backward operational run by canceling the check function. The check function checks the backward operational run based on a signal indicating an on/off state of the clutch mechanism and a signal indicating a traveling direction of the vehicle body determined by the shifting device. The release function maintains the check function off continuously by operating a release switch when the clutch mechanism is engaged, and the signal indicating a traveling direction of the vehicle body indicates forward or neutral.