This paper is concerned with a design of a digital adaptive control system for a nonminimum phase system with an unknown time delay. In the case of continuous time controlled systems having a nonminimum phase characteristic, discrete versions also have the characteristic. Furthermore, if the sampling period is sufficiently small, discretized continuous time minimum phase systems with a relative degree larger than 2 become nonminimum phase systemS. We have proposed a design method of an adaptive control system consisting of a pole placement controller, and series and parallel compensators which compensate unstable zeros. >From experiments of the position control of an underwater pendulum with an undershooting phenomenon, the method is effective to the nonminimum phase systems. Here, we extend the method to the case of unknown time delay systems. The proposed method guarantees the stability and convergency of the adaptive system. In order to verify the validity of the method, simulations of minimum and nonminimum phase systems and experiments of the position control of an underwater pendulum are given. The simulation and experimental results show the high performance of the constructed adaptive control system and the effectiveness of the proposed method.