A coaxial type starter motor comprises a d.c. motor comprising a hollow armature rotary shaft and a front bracket connected to a yoke, an overrunning clutch disposed in the front bracket to transmit a rotational force from the rotary shaft, an output shaft having a rear end portion which is extended into the hollow rotary shaft and is supported by a bearing so as to be slidable forwardly and backwardly, the output shaft having a helical spline connection to a clutch inner in the overrunning clutch to obtain the transmission of a rotational force, and a pinion mounted on the front end portion of the output shaft so as to come to interlocking with the ring gear of an engine when the output shaft is forwardly moved, wherein a clutch outer in the overrunning clutch has an enlarged diameter portion for supporting a roller at its rear end side, a boss portion of a reduced diameter at its front end side, in which the outer periphery of the boss portion is supported by the front bracket by means of a bearing, and the clutch inner is supported by the inner periphery of the boss portion of the clutch outer by means of a bearing.