Provided is an automatic transmission capable of reducing friction loss. The automatic transmission is provided with an input shaft (2) coupled with a sun gear (Sa), an output member (3), a first to third planetary gear mechanisms (4, 5, 6), a first engagement mechanism (C1) coupling the input shaft (2) with a ring gear (Rc) releasably, a second engagement mechanism (C2) coupling a carrier (Ca) with a carrier (Cc) releasably, a third engagement mechanism (C3) coupling the carrier (Ca) with a second coupling body (Rb-Sc) releasably, a fourth engagement mechanism (B1) fixing a first coupling body (Ra-Cb) to a transmission case (1) releasably, a fifth engagement mechanism (B2) fixing a sun gear (Sb) to the transmission case (1) releasably, and a sixth engagement mechanism (B3) fixing the ring gear (Rc) to the transmission case (1) releasably.