A focus adjusting apparatus comprises a focus lens device provided in an optical system. The focus lens device comprises a transparent container of a rectangular prism form, a lens member supported in the container and formed of poly(acrylic amide) gel as a polymer gel substance having electro-reactive deformability, and a pair of transparent electrodes, the container being filled with water-aceton mixture solution. The lens member is formed by two lens materials having different proportions of acrylic acid group. A voltage is applied to the transparent electrodes under control of an automatic focusing circuit to cause electric field in the container, whereby the lens member is deformed in a concentric spherical form to form a lens. When the application of the voltage is stopped, the lens member returns to the initial form. A focal distance of the lens member is changed according to degrees of such deformation of the lens member.