To miniaturize a measuring device for optically measuring the vibration of a measured object by bringing it into a contact with the vibrating measured object and to make it lighter, and moreover to simplify its manufacture and operation.

A vibration detecting sensor 10 has a casing 12 vibrated together with a measured object M by bringing it into contact with the vibrating measured object M; a weight 14 accommodated in the casing 12, coil springs 161, 162 for oscillatably supporting the weight 14 to the cage body 12, a reflection plate 18 firmly fixed to the weight 14; and an optical fiber 20 whose one end 201 is fixed to the casing 12 so that a laser beam emitted from the one end 201 may be reflected on the reflection plate 18 to enter the one end 201 again.