An automatic transmission comprising a plurality of speed-change hydraulic clutches selectively operated in dependence of various speeds separately carried by an input shaft and an output shaft, and wherein working oil is supplied from a hydraulic oil source to the hydraulic clutches through (a) fixed oil passages provided in a casing, (b) conduction pipes arranged in coaxially spaced relation within one another in a central bore in the input shaft and/or the output shaft, and (c) communicating passages formed in the input shaft and/or output shaft and connected to the hydraulic clutches. The conduction pipes are respectively connected at one end with the fixed oil passages in a fixed or pivotal manner and at the other end with the communicating passages with seals arranged between the outer peripheries thereof and the inner periphery of the bore in the shaft.