A variable focal length lens device includes: a lens system whose refractive index is variable in response to an inputted drive signal; an image detector for detecting an image of a target object through the lens system; a lens controller for outputting the drive signal and an illumination signal; and a lens operation unit for adjusting a frequency and amplitude of the drive signal outputted by the lens controller and an image-detection timing for the image detector. The lens controller includes a resonance-lock controller for tuning the drive signal to a resonance frequency of the lens system. The lens operation unit includes a resonance-lock operation unit for switchably enabling and suspending the resonance-lock controller.