PURPOSE: To enable observation inside a tank safely even during the operation, by fixing a flange receptacle with a sleeve on a cylindrical body provided with an air supply port and an exhaust port while a sealing body is fitted securely into the body.

CONSTITUTION: Both ends of a flexible sleeve 15 are fixed on a flange 9 with a sleeve and a second flange 11 with a sleeve. A spacer 16 is mounted between the flanges 9 and 11 surrounding a sleeve 15 to form a sealing body with the sleeve 15 and the spacer 16 fixed on the flanges 9 and 11. A flange receptacle 6 with a sleeve is fastened on a hollow body 1 provided with a supply port 4 and an exhaust port 5 for air and the sealing body is fitted securely into the body 1. In observation inside the tank, a fiber scope is inserted into the tank through the flange 11 via the flange receiver 6, the flange 9 and a sleeve 15. Then, a pressure air introduced from the supply port 4 acts on the sleeve 15 through a tiny hole of the spacer 16 to contact the fiber scope. This enables prevention of leakage of a pressure or a fluid from the side of the tank.