A signal processing device includes a Kalman filter that performs Kalman filter processing based on an observation noise and a system noise and outputs a DC component of an input signal as an estimated value; and a monitoring circuit. The Kalman filter outputs an error covariance of the estimated value. The monitoring circuit performs a stop command of observation update processing in the Kalman filter, based on a result of determination processing based on the error covariance, with respect to a signal level corresponding to the input signal.