In this paper, we propose a new multicast technique in wormhole-switched bidirectional multistage interconnection network(B-MIN) for constructing large-scale multicomputer. In addition to point-to-point communication among processing nodes, multicast communication is an important issue for the performance of multicomputers. Multicast communication in which the same packet is delivered from a source node to an arbitrary number of destination nodes, is fundamental in supporting collective communication such as broadcast, reduction, and barrier synchronization. In wormhole-switched B-MIN, to support multicast efficiently without deadlock, we propose a two-phase multicast algorithm which takes only two transmissions to perform a multicast to the desired destinations. We encode a header as a cube and adopt the rotated-upper-input-port-first scheme as arbitration mechanism at output ports. And then, we show the blocking-freedom and deadlock -freedom of the proposed algorithm and evaluate the performance by simulation. The proposed multicast algorithm makes a significant improvement in terms of multicast latency.