To make it possible to easily and continuously control focal length by applying an adequate external force on a lens body consisting of a solid material having transmissibility and elasticity.

This focal length variable lens has a disk-shaped lens body 10 made of gelled soft polyethylene which is an example of the solid material having the transmissibility and elasticity and a cap nut 20 and bolt 22 which are an example of a stress generating member. The cap nut 20 and the bolt 22 screwed thereto are capable of holding and pressurizing the peripheral edge of the lens body 10. The external force is applied to the lens body 10 of this lens by screwing the bolt 22 in an arrow A direction into the cap nut 20 to elastically deform the lens body, thereby, the lens body can be made into the convex lens shape. The focal length of the convex lens 10' obtainable at this time is changeable by controlling the elasticity of the lens body 10 and the distance at which the bolt 22 is screwed in. The continuous adjustment of the focal length of the convex lens 10' is thus possible.