An eccentric oscillating speed reducer includes an input rotor, a first rotor, a second rotor, a planetary rotor, a flow passage, a contact portion and a radial relief groove. The flow passage includes an axial gap located between the planetary rotor and a contact member. The flow passage conducts a lubricant oil that is forced to flow by a centrifugal force. The contact portion is placed in a drive force transmission path from the planetary rotor to the second rotor. The contact portion includes: a planetary rotor end surface of the planetary rotor located on a side where the second rotor is placed; and a second rotor end surface of the second rotor, which is opposed to the planetary rotor end surface. The radial relief groove forms a portion of the flow passage and extends through a radial width of the planetary rotor end surface.