PROBLEM TO BE SOLVED: To provide an imaging optical system for endoscope which can switch a wide angle observation state and a telephoto observation state and can satisfactorily correct aberration.SOLUTION: The imaging optical system for endoscope comprises: a first negative group a second negative group an aperture diaphragm a third positive group and a fourth positive group in this order from an object side. The first group is constituted of a lens turning its concave surface on an image side. The second group is constituted of a cemented lens including a negative lens and a positive lens turning their concave surfaces to the object side in this order from the object side. The third group is constituted of a cemented lens including a negative lens and a positive lens in this order from the object side. The fourth group includes at least one cemented lens, switches from a wide angle observation state to a telephoto observation state by moving the third group in the direction of approaching the aperture diaphragm along an optical axis, and satisfies the following conditional expression: &ngr<&ngr. In the expression, &ngris the Abbe number of the negative lens of the cemented lens of the third group, and &ngris the Abbe number of the positive lens of the cemented lens of the third group.
