A method for measuring sea waves includes: transmitting ultrasonic waves from an ultrasonic transmitter 2 provided on a buoy 1 into the sea, and receiving the ultrasonic waves from three transponders 3 on the seabed together with the time signals; detecting distances between the transponders 3 and the ultrasonic transmitter 2 based on the propagation time of the ultrasonic waves; high-pass filtering the distance data to extract a short-term fluctuation component; preparing and solving linear equations to determine the displacement in the height direction of the short-term fluctuation component and obtain a wave height.