A strongly interacting massive particle (SIMP) is an interesting candidate for dark matter (DM) because its self-interaction cross section can be naturally strong enough to address the astrophysical problem of small-scale structure formation. A simple model was proposed by assuming a monopole condensation, where composite SIMP comes from a "strongly interacting"U(1)d gauge theory. In the original model, the DM relic abundance is determined by the 3→2 annihilation process via the Wess-Zumino-Witten term. In this paper, we discuss that the DM relic abundance is naturally determined also by a semiannihilation process via a kinetic mixing between the hypercharge gauge boson and the dark U(1)d gauge boson (dark photon). The dark photon can be discovered by LDMX-style missing momentum experiments in the near future.