A tire condition monitoring system includes a transmission device, a receiving device, and a monitoring section. The transmission device includes a sensor, a transmitter, a housing, and a engaged member. The sensor detects, as the tire information, a condition of gas filled in the tire cavity area surrounded with the tire and a rim. The transmitter wirelessly transmits the detected tire information. The housing houses the sensor and the transmitter, including an inner space and a through hole. The inner space is surrounded with a wall of the housing and is separated from the tire cavity area. The through hole passes through the wall. The engaged member is engaged with the through hole to be provided to the housing. The engaged member includes one of an air vent and a communication groove for communicating between the tire cavity area and the inner space.