In an operation amount detecting apparatus, a brake pedal is coupled with an operation rod by a coupling shaft, respective end portions of an elastic member are supported at support positions of a brake pedal by support shafts, the brake pedal is away in a direction orthogonal to an axis line direction of an operation rod with respect to the coupling shaft, an intermediate portion of the elastic member is relatively displaceably coupled by the coupling shaft and a coupling hole, and the elastic member is disposed with output strain sensors which detect an elastic deformation amount of the elastic member and output the elastic deformation amount as a brake operation amount.