Disclosed is a power-transmitting apparatus. A power-transmitting apparatus of the present invention comprises: a pin gear provided with a plurality of pins capable of rolling movement; and a spur gear provided with a plurality of teeth and en-gagingly coupled to the pin gear so as to be relatively movable, wherein the spur gear teeth are formed with a cycloid tooth shape, and a polygonal tooth root space is formed at a tooth root region of the spur gear. As compared to a trochoid tooth shape, the tooth shape of the spur gear engagingly coupled to the pin gear so as to be relatively movable according to the present invention has a greater tooth size so that it can be applied as a cycloid tooth shape capable of maintaining a sufficient tooth contact ratio, and can resolve a reduction in tooth thickness and the wedging of the pins from an undercut at the tooth root region.