A tire condition monitoring system includes a transmission device, a receiving device, and a monitoring section. The transmitting device includes a sensor which detects, as tire information, a condition of gas in the tire cavity area surrounded with a tire and a rim, a transmitter which wirelessly transmits the tire information, and a housing having a wall surrounding the sensor and the transmitter and separating an inner space from the tire cavity area. The housing wall defines a communicating through hole which enables communication between the cavity area and inner space and a selectively non-communicating through hole having a member which is movable between blocking and unblocking positions to impede and enable communication, respectively, between the cavity area and inner space. The receiving device receives the tire information, and the monitoring section determines whether a tire anomaly exists based on the tire information and provides a result.