A method for transmitting an uplink signal of a user equipment (UE) in a wireless communication system includes receiving an uplink (UL) grant from a serving base station and performing UL transmission based on the UL grant or device-to-device (D2D) communication, according to a configuration for D2D communication and a configuration of transmission time interval (TTI) bundling. When bundled UL transmission based on the UL grant and transmission for D2D communication are scheduled in the same TTI, the UL transmission scheduled in the TTI is performed in a first available TTI after the TTI, and, when two bundled UL transmissions according to two different UL grants are scheduled in the same TTI, bundled UL transmission according to the later received UL grant of the two different UL grants is dropped.