A hydraulic four-wheel-drive working vehicle includes a hydraulic pump unit having a hydraulic pump main body of a variable displacement type, a pair of left and right first motor devices driving a left and right first driving wheels, and a pair of left and right second motor devices driving a left and right second driving wheels, wherein each of the pair of first motor devices and the pair of second motor devices includes a hydraulic motor main body forming an HST in cooperation with the hydraulic pump main body, a motor volume adjusting mechanism, and a hydraulic actuating mechanism that operates the motor volume adjusting mechanism, and the hydraulic actuating mechanisms in the pair of first motor devices and the pair of second motor devices are operated and controlled with use of hydraulic fluid from a single hydraulic pressure source.