A reflective optical sensor includes a light emitting element to irradiate a target object with a light beam, a light receiving element to receive the light beam reflected by the surface of the target object, a circuit board on which the light emitting element and light receiving element are mounted, a cover element supported on the circuit board to surround the light emitting element and light receiving element, and a light shielding wall provided in the cover element to be a partition between the light emitting element and light receiving element to prevent a part of the light beam from the light emitting element from leaking to the light receiving element. A layer in a certain thickness is formed on the surface of the circuit board along either or both sides of the light shielding wall.