A light scanning apparatus has a base member, a first engagement portion that engages with a bearing member having a bearing portion of a rotary shaft that rotates a rotational polygon mirror to thereby position the bearing member, a second engagement portion that engages with a bearing member having a bearing portion of a rotary shaft that rotates a rotational polygon mirror to thereby position the bearing member, and a positioning portion in which the first engagement portion and the second engagement portion are integrally formed, the positioning portion being provided on the base member. With this structure, a common base member can be used in optical units, and even when rotational polygon mirrors are supported on different surfaces of the base members of optical units, a difference in precision of support can be made small.