An optional-attachment-operating direction switching valve is provided in a part of an unloading passage which part is on the upstream of an arm-operating direction switching valve and a bucket-operating direction switching valve. The hydraulic circuit is arranged such that the optional-attachment-operating direction switching valve receives pressure oil from a pressure oil supply passage which branches from the part of the unloading passage on the upstream of the optional-attachment-operating direction switching valve, the arm-operating direction switching valve receives pressure oil from a pressure oil supply passage which branches from a part of the unloading passage which part is between the optional-attachment-operating direction switching valve and the arm-operating direction switching valve, and the bucket-operating direction switching valve receives the pressure oil from a pressure oil supply passage which branches from a part of the unloading passage which part is between the optional-attachment-operating direction switching valve and the bucket-operating direction switching valve.