An imaging optical system comprising, sequentially from an object side toward an image side: a first lens group (G1) having one of a positive refractive power and a negative refractive power; an aperture stop (S); and a second lens group (G2) having a positive refractive power, wherein the first lens group (G1) 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 (G2) 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 (G2), c32a is an on-axis curvature of an image-side surface of the plastic lens of the second lens group (G2), c31b is an off-axis curvature of the object-side surface of the plastic lens of the second lens group (G2), c32b is an off-axis curvature of the image-side surface of the plastic lens of the second lens group (G2); 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 (G1) having one of a positive refractive power and a negative refractive power; an aperture stop (S); and a second lens group (G2) having a positive refractive power, wherein the first lens group (G1) 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 (G2) 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 (G2), c32a is an on-axis curvature of an image-side surface of the plastic lens of the second lens group (G2), c31b is an off-axis curvature of the object-side surface of the plastic lens of the second lens group (G2), c32b is an off-axis curvature of the image-side surface of the plastic lens of the second lens group (G2); 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 (G2), c32a is an on-axis curvature of an image-side surface of the plastic lens of the second lens group (G2), c31b is an off-axis curvature of the object-side surface of the plastic lens of the second lens group (G2), c32b is an off-axis curvature of the image-side surface of the plastic lens of the second lens group (G2); 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. <EOS>