Cluster of workstation or personal computers connected with high speed network has become one of major architectures of distributed memory parallel computers. However, software on the cluster environment is still not improved in performance. The distributed shared memory can be a solution of programming style on distributed memory parallel system including clusters because we know from experiences that shared memory model ease programming. However, it must be implemented with care for the performance problem where there is no hardware support for shared memory system. Another, but serious problem is the portability. In this paper, we discuss the design and implementation of portable distributed shared memory system. Our shared memory system is based on PVM in consideration of portability. Our contributions in this paper is the design and implementation of portable shared memory system on the cluster environment using faithful implementation of active messages fully in software, together with an enhancement of PVM to support active messages.