A rear attachment lens that is capable of being detachably attached to a main lens system at an image side of the main lens system to lengthen a focal length from a focal length of the main lens system itself, the rear attachment lens consisting of: a first lens unit having a positive refractive power and consisting of either two lenses including a positive lens and a negative lens or a set of a single lens and a cemented lens arranged in order from an object side to the image side, the set of the single lens and the cemented lens including a positive lens and a negative lens; a second lens unit positioned on the image side of the first lens unit and consisting of either a cemented lens including a positive lens and a negative lens or a set of a single lens and a cemented lens arranged in order from the object side to the image side, the set of the single lens and the cemented lens including a positive lens and a negative lens; a third lens unit positioned on the image side of the second lens unit and consisting of either a positive lens or a positive lens and at least one negative lens; a fourth lens unit positioned on the image side of the third lens unit and consisting of either two lenses including a positive lens and a negative lens or a set of a cemented lens and a single lens arranged in order from an object side to the image side, the set of the cemented lens and the single lens including a positive lens and a negative lens, characterized in that the at least one negative lens in the third lens unit, if present, and/or the negative lens included in the fourth lens unit serves as a first negative lens, wherein, when ν d_N1 and Δθ gF_N1 are an Abbe number and a partial-dispersion-ratio difference, respectively, of a material of the first negative lens, f _N1 is a focal length of the first negative lens, and f is a focal length of the rear attachment lens, the following condition is satisfied: 0.00015 < Δ θ gF _ N 1 * f / ν d _ N 1 * f _ N 1 < 0.00620; and at least one of the positive lenses included in the third and fourth lens units (RL3, RL4) serves as a first positive lens, and when ν d_P1 and Δθ gF_P1 are an Abbe number and a partial-dispersion-ratio difference, respectively, of a material of the first positive lens and f _P1 is a focal length of the first positive lens, the following condition is satisfied: 0 < Δ θ gF _ P 1 * f / ν d _ P 1 * f _ P 1 < 0.001. a first lens unit having a positive refractive power and consisting of either two lenses including a positive lens and a negative lens or a set of a single lens and a cemented lens arranged in order from an object side to the image side, the set of the single lens and the cemented lens including a positive lens and a negative lens; a second lens unit positioned on the image side of the first lens unit and consisting of either a cemented lens including a positive lens and a negative lens or a set of a single lens and a cemented lens arranged in order from the object side to the image side, the set of the single lens and the cemented lens including a positive lens and a negative lens; a third lens unit positioned on the image side of the second lens unit and consisting of either a positive lens or a positive lens and at least one negative lens; a fourth lens unit positioned on the image side of the third lens unit and consisting of either two lenses including a positive lens and a negative lens or a set of a cemented lens and a single lens arranged in order from an object side to the image side, the set of the cemented lens and the single lens including a positive lens and a negative lens,