A constant-modulus alloy, which has a low saturation magnetic flux density to provide weakly magnetic properties, a high Young's modulus, a low temperature coefficient of Young's modulus, and high hardness, is provided. A hairspring, a mechanical driving apparatus and a watch and clock, in which the alloy is used, are provided. The alloy consists of Co, Ni, Cr, Mo. and Fe. The alloy is healed and cooled before being subjected to repeated wiredrawing and intermediate annealing, forming a wire with a fiber structure having a fiber axis. The wire is then cold rolled into a sheet and heated to obtain optimal magnetic insensitivity and hardness.