A torque-detecting device having sufficient mechanical strength, capable of performing accurate torque measurement, having excellent workability suitable for mass-production, and capable of avoiding a problem of damage in heat treatment. The torque-detecting device constituted of a portion (3) subjected to torque detection provided on a surface layer portion of a steel torque-transmitting shaft (2), a detecting coil (4) provided opposite the outer periphery of the portion (3) without contact, and detected signal-processing means (5). The portion (3) subjected to torque detection is a magnetostriction material layer constructed from an Al diffusion layer. In the diffusion layer, Al is diffused in the steel material such that its concentration gradually diminishes from the surface toward the center of the shaft.