In an aspect of the present disclosure, a submarine device includes a main tail cable connected to a submarine cable, a first branch tail cable including a first group of optical fibers among a plurality of optical fibers included in the main tail cable, a second branch tail cable including a second group of optical fibers among the plurality of optical fibers, a branch member that couples the main tail cable to the first and second branch tail cables and including therein a through hole for branching the plurality of optical fibers included in the main tail cable into the first group and the second group, and a device main body including a first introduction part for introducing the first branch tail cable into the device main body and a second introduction part for introducing the second branch tail cable into the device main body.