PURPOSE: To eliminate a change in sensitivity due to a car height and a speed, by a method wherein, in a method to measure a running speed through sensing of a vehicle by means of sensors imbedded in a road surface at given intervals, a flux gate type magnetic field sensor and a magnet are employed.

CONSTITUTION: A ring core 1 and a sensor coil 2 are rolled around a permalloy ring core 1, and an output of a low-frequency oscillator 4 is amplified at a device 5. So amplified output is supplied to a coil 3, and as a result, an output is produced from the coil 2 and is inputted to a phase detector 6 through a band-pass filter 5'. The other output of the oscillator 4 is multiplied 2 times by means of a frequency multiplier 7, and is inputted to the phase detector 6 via a phase shifter 8. A magnitude of a magnetic field can be meausured through the reading of the output of the detector 6 via a filter 9. A plural number of said sensors are imbedded in a road surface, and a running speed is obtained from a time difference through the detection of a magnet piece on a vehicle. The sensor has an enough sensitivity against even a vehicle having a height of over 500cm, and thereby a high-precise measurement can be performed.