A signal processing device of a scanning-type distance measurement device repetitively scanning, with use of measurement light, a to-be-monitored region partitioned within a predetermined separation distance at predetermined scan cycles to detect reflected light from an object with respect to the measurement light at each predetermined scan angle, the signal processing device comprising: a distance calculation unit configured to calculate a distance from the scanning-type distance measurement device to the object at the each predetermined scan angle in accordance with delay time and/or a phase difference between measurement light and reflected light; a to-be-monitored object determination unit configured to perform a to-be-monitored object determination process of determining that there is a to-be-monitored object in specific scan angle directions at the N-th scan where the distance calculation unit calculates a distance within the to-be-monitored region consecutively for N number of scans, N being an integer equal to or greater than two, in the specific scan angle directions, when a distance within the to-be-monitored region is calculated in M number of adjacent scan angle directions, M being an integer equal to or greater than two, including the specific scan angle directions each consecutively for N number of scan; and a signal output unit configured to output a to-be-monitored object detection signal when the to-be-monitored object determination unit determines that there is the to-be-monitored object, characterised in that the to-be-monitored object determination unit is configured to regard that a distance within the to-be-monitored region is calculated consecutively for the N number of scans in the scan angle directions at the N-th scan where the distance calculation unit calculates a distance within the to-be-monitored region consecutively for the N number of scans in the specific scan angle directions, when a distance within the to-be-monitored region is calculated in the M number of adjacent scan angle directions including the specific scan angle directions and a distance within the to-be-monitored region is not calculated consecutively for the N number of scans in at least one of the M number of scan angle directions, and perform the to-be-monitored object determination process and the to-be-monitored object determination unit is configured to regard that a distance within the to-be-monitored region is calculated in the scan angle directions at the N-th scan where the distance calculation unit calculates a distance within the to-be-monitored region consecutively for the N number of scans in the specific scan angle directions, when a distance within the to-be-monitored region is not calculated and a distance within the to-be-monitored region has been calculated at a last scan in at least one of the M number of adjacent scan angle directions including the specific scan angle directions, and perform the to-be-monitored object determination process. a distance calculation unit configured to calculate a distance from the scanning-type distance measurement device to the object at the each predetermined scan angle in accordance with delay time and/or a phase difference between measurement light and reflected light; a to-be-monitored object determination unit configured to perform a to-be-monitored object determination process of determining that there is a to-be-monitored object in specific scan angle directions at the N-th scan where the distance calculation unit calculates a distance within the to-be-monitored region consecutively for N number of scans, N being an integer equal to or greater than two, in the specific scan angle directions, when a distance within the to-be-monitored region is calculated in M number of adjacent scan angle directions, M being an integer equal to or greater than two, including the specific scan angle directions each consecutively for N number of scan; and a signal output unit configured to output a to-be-monitored object detection signal when the to-be-monitored object determination unit determines that there is the to-be-monitored object, characterised in that the to-be-monitored object determination unit is configured to regard that a distance within the to-be-monitored region is calculated consecutively for the N number of scans in the scan angle directions at the N-th scan where the distance calculation unit calculates a distance within the to-be-monitored region consecutively for the N number of scans in the specific scan angle directions, when a distance within the to-be-monitored region is calculated in the M number of adjacent scan angle directions including the specific scan angle directions and a distance within the to-be-monitored region is not calculated consecutively for the N number of scans in at least one of the M number of scan angle directions, and perform the to-be-monitored object determination process and the to-be-monitored object determination unit is configured to regard that a distance within the to-be-monitored region is calculated in the scan angle directions at the N-th scan where the distance calculation unit calculates a distance within the to-be-monitored region consecutively for the N number of scans in the specific scan angle directions, when a distance within the to-be-monitored region is not calculated and a distance within the to-be-monitored region has been calculated at a last scan in at least one of the M number of adjacent scan angle directions including the specific scan angle directions, and perform the to-be-monitored object determination process.