A access apparatus is set on instrumentation nozzles penetrating a bottom portion of a reactor pressure vessel, in the reactor pressure vessel filling water. The access apparatus has an arm turned, and a mounting fixture that can move along the arm and having a holding member. A cover apparatus having a guide pipe covers the access apparatus, and is installed on the bottom portion in underwater environment in the reactor pressure vessel. The water below the cover apparatus is drained from the reactor pressure vessel to form an air space below the cover apparatus. A variety of device heads is suspended and lowered in the air space through the guide pipe, and mounted to the holding member of the mounting fixture. Repair or preventive maintenance operation against the instrumentation nozzles is performed by the variety of device heads.