A ball type speed reducer includes: an eccentric disk cam rotating in synchronization with an input-side rotating body; a shaking body fitted relatively rotatably to an outer circumference side of the eccentric disk cam to be shaken; balls rollably housed in a ball holding portion of the shaking body; and a fixing member having a first side face portion facing one side face of the shaking body. Radial grooves for radially guiding the balls are formed in the first side face portion. An output-side rotating body has a second side face portion facing the other side face of the shaking body. An annular corrugated groove for guiding the balls circumferentially in an undulating manner is formed in the second side face portion. The balls are engaged with the radial grooves and the corrugated groove and are rolled inside the radial grooves and the corrugated groove as the shaking body is shaken.