A tsunami monitoring system includes a transmitting antenna configured to radiate a transmission signal to detect a tsunami as a radio wave toward a sea, and a receiving antenna configured to receive reflected waves reflected by the tsunami as a receiving signal. The tsunami monitoring system includes a signal generator circuit configured to generate the transmission signal having a predetermined frequency, a signal processor portion configured to generate a beat signal of a frequency difference between the transmission signal and the receiving signal, and a wave height estimator portion configured to divide a radio wave radiation region into a plurality of regions, calculate a flow velocity of a sea surface of the tsunami for each region on the basis of the beat signal, and estimate a wave height of the tsunami from a calculated flow velocity.