We consider device-to-device (D2D) direct communication underlying cellular networks where the D2D link reuses the frequency resources of the cellular downlink. In this paper, we propose a linear precoder design scheme for a base station (BS) and D2D transmitter using the weighted sum-rate of the cellular downlink and D2D link as a cost function. Because the weighted sum-rate maximization problem is not convex on the precoding matrices of BS and D2D transmitters, an equivalent mean-squared error (MSE) minimization problem which is convex on the precoding matrices is proposed by introducing auxiliary matrices. We show that the two optimization problems have the same optimal solution for the precoding matrices. Then, an iterative algorithm for solving the equivalent MSE minimization problem is presented. Through a computer simulation, we show that the proposed scheme offers better weighted sum-rate performance that a conventional scheme.