A three-dimensional image pickup optical system for forming two different images having a parallax alternately in time sequence on a single image pickup element, includes, in order from object side: a front system including a pair of negative 1f-lens units, a pair of an optical path deflecting units, and a combining unit for combining two optical paths on the same optical axis; and a rear system including a positive 1b-lens unit which does not move for magnification-varying, a lens unit moving for magnification-varying, and a stop, in which: the front system includes a pair of light quantity control units disposed in optical paths for the eyes on the object side of the combining unit, for switching left and right subject images alternately in a time sharing manner; and power arrangement of the 1f-lens units and the 1b-lens unit is set appropriately.