To provide a fitting mechanism capable of detachably attaching even an auxiliary optical member which is heavy like a magnifier for medical use to spectacles in a stable state.

A clip 1 as the fitting mechanism is for detachably fitting a holder 18 holding magnifiers 36 and 37 to spectacles in front of the lenses of the spectacles 20, and has a fitting part 7 for fitting the holder 18, pairs of left and right clamp parts 2a and 2b, and 3a and 3b elastically clamping the pair of left and right spectacle lenses from front and rear directions respectively, and abutting rods 6a and 6b which have at their tips abutting parts abutting against the front sides of the spectacle lenses outside the clamp parts 2a and 2b, and 3a and 3b respectively.