PURPOSE: To sufficiently monitor the overload of a brake device by measuring the deformation quantity and pressure generated to a brake drum at the time of brake operation, and transmitting an alarm signal in the case of the measured value exceeding a deformation characterizing curve or the higher value.

CONSTITUTION: An electronic evaluation device 12 is stored with a characterizing curve related to the pressure of a rotation angle (h) of a brake drum 1 heated to permissible temperature T, converted into the electric value. The deformation characterizing curve 11 specifies two ranges in a h/p coordinate areas, that is, the permissible temperature T or lower of the drum 1 in a lower range of the curve 11 and the permissible temperature T or higher of the drum 1 in an upper range of the curve 11. The evaluation device 12 is connected on its input end side to a deformation sensor 5 and a pressure sensor 8, and compares each detected rotation angle hist with the value hPist, T of the curve 11 corresponding to the existing pressure Pist and transmits an alarm signal in the case of the value hist being the value of the curve 11 or larger.