In distributed query processing, the semijoin has been used as an effective operator in reducing relations referenced in the query to reduce the total amount of data transmission. In this paper, we propose a new relational algebra operator, 2-way semijoin, which is an extended version of the semijoin, for more cost- effective distributed query processing. The 2-way semijoin is compared to the semijoin in terms of the reduction power and the propagation of reduction effects. We show that the 2-way semijoin has more reduction power than the semijoin and that the propagation of reduction effects by the 2-way semijoin is further than by the semijoin.