An imaging optical system comprising, sequentially from an object side toward an image side: a first lens group having one of a positive refractive power and a negative refractive power; an aperture stop; and a second lens group having a positive refractive power, wherein the first lens group consists of, sequentially from the object side toward the image side: a negative lens having a meniscus shape; and a positive lens having a meniscus shape and formed of a plastic lens, and wherein the second lens group consists of three lenses, including a plastic lens, and characterised in that: the imaging optical system comprises no more than five lenses with refractive power; and the imaging optical system satisfies Conditional Expressions and as follows: c 31 a > c 32 a, and c 31 b < c 32 b, where c31a is an on-axis curvature of an object-side surface of the plastic lens of the second lens group, c32a is an on-axis curvature of an image-side surface of the plastic lens of the second lens group, c31b is an off-axis curvature of the object-side surface of the plastic lens of the second lens group, c32b is an off-axis curvature of the image-side surface of the plastic lens of the second lens group; and off-axis curvature represents a local curvature at a lens height of the smaller of an effective diameter at an outermost end of an object-side surface and an effective diameter at an outermost end of an image-side surface. a first lens group having one of a positive refractive power and a negative refractive power; an aperture stop; and a second lens group having a positive refractive power, wherein the first lens group consists of, sequentially from the object side toward the image side: a negative lens having a meniscus shape; and a positive lens having a meniscus shape and formed of a plastic lens, and a negative lens having a meniscus shape; and a positive lens having a meniscus shape and formed of a plastic lens, and wherein the second lens group consists of three lenses, including a plastic lens, and characterised in that: the imaging optical system comprises no more than five lenses with refractive power; and the imaging optical system satisfies Conditional Expressions and as follows: c 31 a > c 32 a, and c 31 b < c 32 b, where c31a is an on-axis curvature of an object-side surface of the plastic lens of the second lens group, c32a is an on-axis curvature of an image-side surface of the plastic lens of the second lens group, c31b is an off-axis curvature of the object-side surface of the plastic lens of the second lens group, c32b is an off-axis curvature of the image-side surface of the plastic lens of the second lens group; and off-axis curvature represents a local curvature at a lens height of the smaller of an effective diameter at an outermost end of an object-side surface and an effective diameter at an outermost end of an image-side surface. the imaging optical system comprises no more than five lenses with refractive power; and the imaging optical system satisfies Conditional Expressions and as follows: c 31 a > c 32 a, and c 31 b < c 32 b, c31a is an on-axis curvature of an object-side surface of the plastic lens of the second lens group, c32a is an on-axis curvature of an image-side surface of the plastic lens of the second lens group, c31b is an off-axis curvature of the object-side surface of the plastic lens of the second lens group, c32b is an off-axis curvature of the image-side surface of the plastic lens of the second lens group; and off-axis curvature represents a local curvature at a lens height of the smaller of an effective diameter at an outermost end of an object-side surface and an effective diameter at an outermost end of an image-side surface.