A received beam amount acquisition section 60 acquires a received beam amount per unit time of a beam receiver 52 at certain sampling time intervals. A beam shielding detection section 66 detects presence or absence of beam shielding of a monitoring beam B by determining whether or not the acquired received beam amount per unit time of the beam receiver is lower than a determination threshold Th for detecting the presence of beam shielding of the monitoring beam B. A detection sensitivity of the beam shielding detection section 66 is configured to be switched from a normal sensitivity mode to a high sensitivity mode when bending is initial processing based on a predetermined processing program, and an acquired height position of the beam receiver 52 is lower than a predetermined second height position LP2.