In this paper, we propose a power allocation method for the direct/cooperative amplify-and-forward (AF) relay switched SC-FDMA using spectrum division/adaptive subcarrier allocation (SDASA). The cooperative relaying is used only when it can achieve higher channel capacity than the direct communication. In the proposed power allocation method, transmit power is adaptively allocated to mobile terminal (MT) and relay station (RS) according to the channel conditions of MT-RS link and RS-base station (BS) link when the use of cooperative relay is selected. We evaluate the achievable channel capacity of the proposed power allocation method by Monte-Carlo numerical computation method. It is shown that the proposed power allocation method can achieve almost the same channel capacity as the grid search method.