PURPOSE: To detect an abnormality of a pressure sensor by detecting the pressure of a ram operating time, and comparing with an expecting pressure which is expected so as to be generated in the ram operating time is calculated and stored in advance.

CONSTITUTION: A pressure generating in an operating time of a ram of a bending machine controlling the bending of a steel plate is detected with a pressure sensor 19, and every kind of machining condition is preliminarily inputted. Then, about each kind of machining condition, a ram operating position and a pressure being expected to be generated in the ram operating time are calculated in advance and stored in a memory part 25. The pressure sensor 19 detects the pressure in the ram operating time. An encoder 31 detects the ram position. Both are compared with the data of the memory part 25, and whether the abnormality is or not is checked. Before the pressure sensor 19 is perfectly troubled, whether the abnormality is or not can be detected, and the degraded article can be prevented from manufacturing.