To cancel an error in GPS wave propagation time and calculate a relative position of a car running nearby to the own car position with good accuracy, a relative car positioning system using car communication includes a car communication transceiver, a GPS receiving device, a GPS information transmission/reception device, and a relative positioning device. A relative position is determined by determining a GPS wave propagation time difference from a GPS satellite, a wave of which is received by the car running nearby and the own car in common. The difference between the data of GPS wave propagation time at the car running nearby and the data of GPS wave propagation time at the own car is calculated, and if the number of differences of the GPS wave propagation time is obtained by more than three pieces, a relative position of the car running nearby is calculated by solving simultaneous equations expression the relative position as an unknown quantity.