To provide an imaging optical system having a small/thin optical system and satisfactorily corrected various aberration typically chromatic aberration, and to obtain a clear and sharp image while preventing the generation of color blurring in an electronic imaging device.

In the imaging optical system, a positive lens group is disposed at an object side from an aperture stop. In three regions, i.e. a region defined by straight-lines at lower and upper limit values in a range of conditional expression (1) 0.6520gF10.7620 when setting a straight-line expressed by gF1=1d1+gF1 (1=-0.00566), in an orthogonal coordinate system with an abscissa axis d1 and an ordinate axis gF1, a region defined by straight-lines at lower and upper limit values in a range of conditional expression (2) 2.0b12.4 (nd11.3) when setting a straight-line expressed by nd1=a1d1+b1 (a1=-0.0267), in an orthogonal coordinate system with an abscissa axis d1 and an ordinate axis nd1, and a region defined by conditional expression (3) 10d135, gF1 or the like of at least one lens LA constituting the positive lens group is included, where gF1 is a partial dispersion ratio of the lens LA, d1 is the Abbe number of the lens LA, and nd1 is the refractive index of the lens LA at the d-line.