An electric motor drive device (1) includes a motor (11), a speed reducing part (31) to reduce speed of a rotation outputted from the motor (11) and transmit it to the side of a wheel, and a connection/disconnection part (21) to disconnect or connect rotation transmission of the speed reducing part, a motor side rotation member (18) serving as an input shaft to input the rotation from the motor (11), an output shaft (34) to output the rotation to the side of the wheel, a double shaft (33) arranged coaxially with the input shaft and including a tube-shaped outer side rotation shaft (33o) and an inner side rotation shaft (33i) inserted into the outer side rotation shaft.