The present invention offers a power steering device for a four-wheel drive, which includes: a first self-aligning torque estimation unit for estimating a self-aligning torque generated in a current vehicle behavior as a first self-aligning torque; a second self-aligning torque estimation unit for estimating a self-aligning torque that is to influence a subsequent vehicle behavior as a second self-aligning torque; a self-aligning torque change amount calculation unit for calculating a difference of the second self-aligning torque from the first self-aligning torque as a self-aligning torque change amount; and an assisting force correction unit for determining and outputting a corrected value of an assisting force to be generated for a steering operation based on the self-aligning torque change amount. Therefore, even when the self-aligning torque changes as the distribution of the front and rear driving power is variably controlled, a corresponding assisting force is generated, maintaining a natural steering feeling without causing an unpleasant sensation to the driver.