To provide a water level measuring device utilizing a radio wave capable of measuring accurately the distance to the water surface at a short distance.

This water level measuring device is constituted so that a beat signal is detected by orthogonal detection, and that a complex data series wherein a data series of a same-phase component of the beat signal is used as a real part and a data series of an orthogonal component of the beat signal is used as an imaginary part is subjected to discrete Fourier transform. Hereby, the distance from an antenna to the water surface at a short distance can be measured accurately.