To provide a time counting system for competition by which time for each competitor can properly be counted by a simple constitution.

A square loop coil 12 is properly arranged on a runway on which a competitor runs. When a sinusoidal wave is fed from a magnetic field generator 11, the loop coil 12 generates an AC magnetic field on the coil. A radio tag 20 is arranged in such a manner that the detecting direction of a magnetic field detecting coil may become approximately in parallel to the runway surface, and a magnetic field strength distribution in which the magnetic field strength becomes smaller is detected at the center on the loop coil 12. Then, the radio tag 20 judges the arrival at a time counting line L from the detection of the pole changing point of the magnetic field, and time at that point is specified as counted time. The radio tag 20 transmits the specified counted time and so forth to an information receiving device 13.