The object of this invention is to provide a relay unit and a housing for that unit to be used when a number of relay switches are installed side by side in a circuit to switch a power supply circuit on and off. A detecting value expressing the quantity of light received of the detecting light is divided by a reference value expressing the quantity of light received of the reference light, to obtain a computed value, based on which performed is identification of plastic forming the object to be identified 4. A bumper may be coupled to each of the four opposing corners. The hydrogen evolved is used to determine the concentration of the reducing agent present in the sample.