A radar device includes a transmission antenna and a reception antenna having a plurality of antenna elements. The radar device switches the antenna elements in synchronization with a modulation cycle, thereby obtaining a reception signal. At this time, the radar device obtains the reception signal by switching the antenna elements using a first measurement phase and a second measurement phase having different switching cycles as one set. The radar device calculates an azimuth sine value sin θ1 from the reception signal in the first measurement phase and also calculates an azimuth sinusoidal value sin θ2 from the reception signal in the second measurement phase. Then, the radar device calculates a relative velocity V from the azimuth sine value sin θ1, the azimuth sine value sin θ2, an interval time difference Δt between switching cycles, and an inter-antenna element spacing d.