An in-cabin communication system performs wireless communication between a vehicle and a portable terminal carried by an occupant. The in-cabin communication system includes: a vehicular device mounted to the vehicle; and a leaky coaxial cable that is connected to the vehicular device, and that outputs an electromagnetic wave having a predetermined wavelength according to a command from the vehicular device. The leaky coaxial cable is disposed inside a steel plate of a body of the vehicle. The leaky coaxial cable is disposed at a predetermined distance, which corresponds to an integer multiple of a half-wavelength of the predetermined wavelength, from the steel plate.