An automatic transmission for five forward speeds equipped with a main transmission mechanism and an auxiliary transmission mechanism. The auxiliary transmission mechanism includes a multi-disc clutch and a hydraulic servo therefor arranged in a first planetary gear set. The first planetary gear set includes a first ring gear connected to an output unit of the main transmission mechanism and a first carrier connected to an output shaft and further to a first sun gear through a multi-disc clutch and a hydraulic servo for the former. The hydraulic servo for the multi-disc clutch is connected to an integral casing through a band brake and is arranged around the outer circumference of the hydraulic servo for the multi-disc clutch. A second planetary gear set is arranged on the hydraulic servo for the multi-disc clutch and includes a second ring gear connected to the output shaft, a second carrier connected to the integral casing through a multi-disc brake and a second sun gear connected to the first sun gear through the cylinder of a hydraulic servo for the multi-disc clutch. The rear casing forms part of the cylinder of the hydraulic servo for the multi-disc brake. The auxiliary transmission mechanism has a compact three-speed construction to provide five forward speeds in cooperation with the main transmission mechanism. The five-speed automatic transmission can be easily modified merely by removing the second planetary gear set and the brake for the 1st speed to provide a four-speed transmission at a reasonable cost due to the sharing of parts in common.