A tire condition monitoring system stores information representing a number of dual wheels having two tires arranged closely adjacent to each other on, for example, a commercial vehicle. The system measures internal air temperatures of the tires on all of the wheels, and stores temperatures measured when a system reset is executed as reference temperatures. The system determines calculated values based on the temperatures measured for each tire until a measured temperature exceeds the corresponding reference temperature by 20 degrees Celsius or more. The system arranges the calculated values in an order and determines that a number of the calculated values high in the order are calculated values of temperatures of tires mounted on the inside wheel. The number of calculated values corresponding to a dual wheel is equivalent to the number of dual wheels. Thus, the system determines which tires are mounted on an inside wheel.