To highly accurately measure water levels without being affected by the temperature gradient of the ground, subterranean heat, etc., by removing the effects of presser changes associated with temperature changes when measuring ground water levels through the use of a pressure-type sensor.

A pressure sensor unit 9 for measurement and a pressure unit 11 for correction are inserted in the same measuring hole 5. In the pressure sensor unit 9, a pipe 6 for measurement with an open lower end is inserted in an underground water tank 3 in the measuring hole 5, and a pressure sensor 8 for measurement is mounted to the upper end part of the pipe 6 for measurement. In the pressure unit 11 for correction, a pressure sensor 13 for correction is mounted to the upper end of a pipe 12 for correction with a closed lower end, After the opening/closing means 14 of the pipe 6 for measurement and pipe 12 for correction are opened to perform zero-point setting, a water level initial value H0, measurement pressure initial value P0, and correction pressure initial value Q0 are measured and inputted to a processor unit 25. When computing a water level H from the difference between pressure P at measurement and the measurement pressure initial value P0, computation is performed on the basis of the pressure from which a correction value obtained from the difference between correction pressure Q at measurement and the correction pressure initial value Q0 is subtracted.