A fish cleaning device includes a base having an upper surface and a lower surface, one or more arms, each having a first and second end. LEDs are used as light sources and reference detectors are included to control the output of the LEDs. The imaging optical system includes reflection-type off-axial optical elements, each of which has an outer shape whose lengths in directions orthogonal to each other are different from each other and whose thickness is smaller than a length of the outer shape in a short-direction thereof and is made of a resin material. A method of operating the fuel system to purge fuel vapor is also disclosed.