Provided is a mechanism for adjusting the amount of light whereby it is possible to prevent the generation of a difference in transparency seen in the radial direction in optical flux having a prescribed beam diameter, while still making it possible to vary the dimming rate. A mechanism (10) for adjusting the amount of light comprises: an incidence side filter member (11) which comprises a flat incidence face, and the transparency of which varies continuously, as seen in the direction along the incidence face; and an emission side filter member (12) which comprises a flat emission face, and which has an optical concentration gradient equal to that of the incidence-side filter member (11). The direction of continuous variation of the transparency with respect to the optical flux that is incident from the incidence face of the incidence side filter member (11) is defined as the position adjustment direction (P). The direction of reduction of transparency of the emission-side filter member (12) is opposite to the direction of reduction of transparency in the incidence-side filter member (11), and the emission-side filter member (12) is arranged in parallel with the incidence-side filter member (11), in the position adjustment direction (P), and can be moved relative to the incidence-side filter member (11) in the position adjustment direction.