Road surface frictional coefficient estimation device, driving force distribution control device and four-wheel-drive vehicle

A road surface frictional coefficient estimation device (32) includes: a first straight travel determination unit (321) configured to determine whether or not a vehicle travels straight based on a rotational speed difference between a plurality of wheels of the vehicle; a second straight travel determination unit (322) configured to determine whether or not the vehicle travels straight based on a steering angle of the vehicle; a selection unit (323) configured to select one of determination results of the first straight travel determination unit (321) and the second straight travel determination unit (322); and a frictional coefficient estimation unit (324) configured to estimate a frictional coefficient of a road surface on which the vehicle travels when the one of the determination results selected by the selection unit (323) indicates that the vehicle travels straight. A driving force distribution control device (3) mounted on a vehicle including a pair of main drive wheels and a pair of auxiliary drive wheels comprises a respective road surface frictional coefficient estimation device (32). A four-wheel-drive vehicle comprises a respective driving force distribution device (3).