The present invention relates to a radio communication system which supports device-to-device (D2D) communication. More particularly, the present invention relates to a method for transmitting a D2D semi-persistent scheduled (SPS) signal in a radio communication system which supports D2D communication, and an apparatus therefor, the method comprising the steps of: receiving priority information of a D2D SPS signal from a base station; determining whether to send the D2D SPS signal or a signal intended for the base station, on the basis of the priority information, in a case where there is a clash, in a particular sub-frame, between the transmission of the D2D SPS signal and the signal intended for the base station; and transmitting the D2D SPS signal to a second terminal if the priority information of the D2D SPS signal has a higher priority than the signal intended for the base station.