An imaging optical system comprising: a first lens having a concave image side surface and having a negative refractive power; a second lens having a convex object side surface and having a positive refractive index; a third lens having a concave object side surface and having a negative refractive power; a fourth lens having a convex image side surface and having a positive refractive power; and a fifth lens having a convex object side surface and having a positive refractive power in this order from an object side to an image side, the imaging optical system being configured by six or less lenses in total, the first to the fifth lenses being located closest to the object side, characterised in that the imaging optical system satisfies the following conditional expressions: 1.94 < n 2 < 2.20 ; and 15.0 < v 2 < 20.0 , where n 2 represents a refractive index of a lens material of the second lens at d-line, and v 2 represents an Abbe number of the lens material of the second lens at d-line, an Abbe number v of a lens material at d-line being defined by v = n d − 1 / n F − n C , where n C , n d , and n F represent refractive indices of the lens material at C-line, d-line, and F-line, respectively; in that the second lens has a concave image side surface, and the imaging optical system satisfies the following conditional expression: 0.00 < f / r 22 < 0.40 , where f represents a focal length of the entire imaging optical system, and r 22 represents a radius of curvature of the image side surface of the second lens; and and in that the first lens has a concave object side surface, and the imaging optical system satisfies the following conditional expression: − 10.0 < r 11 / f < − 1.0 , where r11 represents a radius of curvature of the concave object side surface of the first lens, and f represents a focal length of the entire imaging optical system. a first lens having a concave image side surface and having a negative refractive power; a second lens having a convex object side surface and having a positive refractive index; a third lens having a concave object side surface and having a negative refractive power; a fourth lens having a convex image side surface and having a positive refractive power; and a fifth lens having a convex object side surface and having a positive refractive power in this order from an object side to an image side, the imaging optical system being configured by six or less lenses in total, the first to the fifth lenses being located closest to the object side, characterised in that the imaging optical system satisfies the following conditional expressions: 1.94 < n 2 < 2.20 ; and 15.0 < v 2 < 20.0 , where n 2 represents a refractive index of a lens material of the second lens at d-line, and v 2 represents an Abbe number of the lens material of the second lens at d-line, an Abbe number v of a lens material at d-line being defined by v = n d − 1 / n F − n C , where n C , n d , and n F represent refractive indices of the lens material at C-line, d-line, and F-line, respectively; in that the second lens has a concave image side surface, and the imaging optical system satisfies the following conditional expression: 0.00 < f / r 22 < 0.40 , where f represents a focal length of the entire imaging optical system, and r 22 represents a radius of curvature of the image side surface of the second lens; and and in that the first lens has a concave object side surface, and the imaging optical system satisfies the following conditional expression: − 10.0 < r 11 / f < − 1.0 , where r11 represents a radius of curvature of the concave object side surface of the first lens, and f represents a focal length of the entire imaging optical system. <EOS>