A variable focus optical element comprises plural elastic members superposed in the axial direction and a deforming member having an aperture to cause the elastic member to protrude therefrom or descend therein thereby deforming the surface of the elastic members, and the modulus of elasticity of an elastic member at the protruding side is selected larger than that of another elastic member adjacent to first-mentioned elastic member at the protruding side. A focus detecting device comprises an imaging optical system having a variable-focus optical element, an image sensor provided at a determined image plane of the imaging optical system or an optically equivalent position, a sharpness detecting device for detecting the sharpness of the image from image signals obtained from the image sensor, and a device for varying the refractive power of the variable-focus optical element, and the focus state of the image on the image sensor is detected by comparing the sharpnesses detected by the sharpness detecting device at plural refractive powers of the variable-focus optical element.