PURPOSE: To detect an approximate mean temperature of a brake drum with a simple constitution by calculating an approximate mean temperature of a component member of the brake drum from a deformation characteristic curve related to a temperature by an approximate component based on thermal deformation.

CONSTITUTION: Pressing force is applied to a brake drum 1 through a brake can 3 and brake pieces 2, 18 by actuation of a brake cylinder 7. A turning angle (h) of this brake can 3 is detected by a deformation sensor 16, pressure in the brake cylinder 7 is detected by a pressure sensor 8, and an electronic evaluating device 12 determines an a average value of approximate temperatures of the brake drum 1 at starting time and finishing time of brake operation from a time limit element 10 from a deformation characteristic curve 11 related to a temperature of the stored turning angle (h) and a characteristic curve 13 related to an elastic modulus of a material of the brake drum 1 to thus calculate a mean approximate temperature of the brake drum with the simple constitution.