A deforming mechanism for deforming a transmissive optical element comprises a rotation member configured to hold the optical element and to rotate around an axis parallel to a tangential line of a circumference of the optical element at a portion where the rotation member holds the optical element, so as to deform the optical element by the rotation, a torque generating unit configured to generate a torque to rotate the rotation member around the axis, a holding base, and an elastic member connecting the holding base to the torque generating unit.