A sensor includes a transmit antenna, a receive antenna, circuitry, and a memory. The transmit antenna includes N transmit antenna elements each transmitting a transmit signal. The receive antenna includes M receive antenna elements each receiving N receive signals including reflection signals reflected by an organism. The circuitry extracts a second matrix corresponding to a predetermined frequency range from an N×M first matrix representing propagation characteristics between each transmit antenna element and each receive antenna element calculated from the receive signals. The circuitry estimates the position of the organism by using the second matrix, and calculates a radar cross-section value with respect to the organism, based on the estimated position and the positions of the transmit antenna and the receive antenna. The circuitry then estimates the posture of the organism by using the calculated radar cross-section value and information indicating associations between radar cross-section values and postures of the organism.