A sensor includes: a transmitting antenna with N transmitting antenna elements transmitting transmission signals; a receiving antenna with M receiving antenna elements, each receiving N received signals including a reflection signal generated by the living body reflecting part of the N transmission signals; a circuit; and a memory. The circuit extracts a second matrix corresponding to a specified frequency range from an N×M first matrix calculated from each received signal and indicating a propagation property between each transmitting antenna element and each receiving antenna element, estimates the position where the living body is present using the second matrix, calculates a radar cross-section (RCS) value of the living body based on the estimated position and the positions of the transmission and receiving antennas, and estimates the motion of the living body using the calculated RCS value and information indicating correspondence between RCS values and motions of the living body.