Problem to be SolvedTo provide an underwater communication system in which no holes need to be drilled in a shell body of a hermetically-sealed structure whose object is to operate underwater.SolutionThe underwater communication system includes: a hermetically-sealed structure 2 including a shell body 5 having a watertight structure and that is disposed underwater; a transmission unit 9 that is arranged in the hermetically-sealed structure 2 and that can perform wireless transmission; a non-conductive propagation medium 4 that includes one end portion being brought into contact with the outside of the shell body 5 having the watertight structure without making a hole therein; and a reception unit 10 that receives an electrical signal wirelessly transmitted from the transmission unit 9 from the other end portion of the propagation medium 4 that is located distally from the shell body 5.