It is important to drive an IPM motor system stably. However, unstable oscillation at the DC input has been reported. According to the impedance method, the stability is assured when the input admittance of the drive system is passive or resistive. In this paper, we propose a passive admittance control method utilizing a Lyapunov-based method. The control principle is first described and then a design example is detailed. It is shown that the proposed method achieves passive frequency characteristics for the input admittance. Its stable operation behaviors were investigated both by simulation and experimentally.