To project a good image even on a curved screen by using liquid in which a design value of index of refraction in the case projecting an image onto a flat screen is different by a specified value of more under the condition of the same wave length and temperature in an optical connection part.

Liquid 9 is filled in the inside space 8 of an optical connection part 4. As the liquid 9, a glycol mixture solution having an index of refraction of about 1.44 usually in the conditions of e-rays and 25°C is used in the case projecting an image onto a flat screen, but in projection onto a curved screen, good focus on the whole screen can not be obtained. Therefore, liquid having an index of refraction different 0.01 or more from that of the glycol mixture solution is used. More specifically, a radius of curvature of a concave lens constituted with the liquid 9 filled in the inside space 8 of the optical connection part 4 is about 63 mm, silicone oil having dimethyl polysiloxane structure having an index of refraction of 1.40 is used as the liquid 9, and a dome-shaped screen having a radius of about 3 m is used.