An automatic transmission comprising a planetary gear mechanism having a first gear driven from a drive shaft, a second gear engagable with the first gear to transmit speed change output to a driven shaft, and a third gear engagable with the second gear to control the operation thereof. A centrifugally operated clutch operates to directly connect the first gear and second gear when the rotational speed of the driven shaft exceeds a predetermined value, and a ratchet type one-way clutch is disposed between the third gear and a fixed construction member to permit rotation of the third gear in the same direction as that of said drive shaft but prevent rotation in the reverse direction. The ratchet type one-way clutch comprises a ratchet pawl supporting plate integrally connected to the third gear, a supporting member having annular ratchet teeth adjacent the supporting plate and fixedly mounted on the fixed construction member, and at least two ratchet pawls rotatably supported on the supporting plate for movement under centrifugal force to engage and disengage the ratchet teeth. The pawls are spaced on the supporting plate so that the pawls are angularly offset with respect to the teeth whereby the pawls can not engage the ratchet teeth concurrently and preferably the pawls are angularly offset with respect to the ratchet teeth by one-half of the pitch of one tooth.