A pattern detection sensor includes a light emitting element for specular reflection, a light emitting element for diffuse reflection light, and a light receiving element. When turning on the light emitting element for specular reflection to emit light, a light source switching unit keeps off the light emitting element for diffuse reflection light. When turning on the light emitting element for diffuse reflection light to emit light, the light source switching unit keeps off the light emitting element for specular reflection. An offset detection unit detects a difference between a detection timing by specular reflection light and another detection timing by diffuse reflection light as an offset value (mutual detection error).