A motor includes inner-peripheral and outer-peripheral rotors which rotate coaxially and have permanent magnets along a circumference of each rotor; and a phase varying device for relatively rotating the rotors so as to vary a relative phase therebetween. The device has a forward-angle working chamber for relatively rotating the inner-peripheral rotor forward with respect to the outer-peripheral rotor, by using pressure of supplied working fluid; a backward-angle working chamber for relatively rotating the inner-peripheral rotor backward with respect to the outer-peripheral rotor, by using pressure of supplied working fluid; a passage switching valve for performing distribution with respect to supply and drainage of the working fluid between the working chambers in accordance with the position of a spur; and an electromagnetic pressure control valve for controlling pressure of the working fluid, and for controlling the position of the spur in the passage switching valve based on the controlled pressure.