An imaging optical system includes sequentially from an object side a front group configured to include a positive lens disposed at a position nearest a diaphragm; the diaphragm; and a rear group configured to include a negative lens disposed at a position nearest the diaphragm. The imaging optical system satisfies a conditional expression (1) 0.27≦|θ3/θ2|≦1.80, where θ3 represents temperature-dependent variation of relative refractive index for d-line of the negative lens and θ2 represents temperature-dependent variation of relative refractive index for d-line of the positive lens.