An observation system having a positive refracting power as a whole comprises a first unit G1 with a positive refracting power and a second unit G2 and forms an exit pupil for observation of an electronic image displayed on an image display element 5. The strainer wall structure has an inlet side and an outlet side through which cooling water is introduced and discharged, respectively, and includes a body having an opening in a direction of the inlet side, closed side surfaces, and an outlet port disposed at one of the closed side surfaces. When a defective area is found, data is transferred to another area in the recording area, and the defective area is registered as being in a recorded state in a table for managing the recorded state in each of the recording areas. The transceiver section converts an inbound RF signal into the inbound symbol stream and converts the outbound symbol stream into an outbound RF signal. A protecting mechanism extends into the receiving space.