A rotation adjustment mechanism includes a rotation target module supported by a fixation member to be freely rotatable around a first rotary shaft and an adjustment unit supported by the fixation member and including a slider movable along a slide axis pointing in a direction orthogonal to the first rotary shaft. The adjustment unit is supported to be freely rotatable around a second rotary shaft parallel to the first rotary shaft and provided on the fixation member. The rotation target module includes a connection groove. The fixation member includes a cam member. The cam member has a guide surface in an arc-like shape that maintains constant a rotation angle of a contact point where the connection groove and a projection part of the slider contact each other around the first rotary shaft with respect to a unit movement amount of the slider in a direction parallel to the slide axis.