To provide an optical-characteristics-variable optical element, together with an imaging device provided with an optical system including the same, such as a variable focal length lens, a variable shape mirror, and a variable prism, with less power consumption, low operational noise, a short response time, and a simple mechanical structure, to reduce the cost.

There is constituted of an optical system 1 provided with a variable mirror 2, a control electrode 8, a zoom lens 3, an encoder 7, and an image pickup element 4, a video signal processing circuit 5, a CPU 6, an amplifier 9, and a driver 10. The shape variable mirror 2 has a focusing function that changes the focal length of a reflecting surface. An LUT 1 is retrieved with a zooming state as a key, and contrast AF is performed based on a plurality of voltage values for driving the variable mirror 2 which are array data corresponding to the zooming state. Driving of the variable mirror 2 is finished at a position when a high frequency component is maximum.