A light quantity adjusting device for adjusting light quantity of a projector apparatus. The adjusting device includes a substrate having an optical path opening and a blade guide surface having level differences in a direction perpendicular to the optical path; a plurality of blade members arranged at a circumferential edge of the optical path opening for rotating along the blade guide surface in an inclined state with respect to the optical path; and a plurality of pins disposed on the substrate for supporting base end portions of the blade members to rotate around the base end portions for opening and closing the optical path opening. A transmission member is mounted on the substrate for engaging the blade members, and a drive device drives the transmission member to rotate the blade members to open and close the optical path opening.