A driving unit using an electrostrictive element drives mechanical members such as, for example, a diaphragm and a shutter or a shutter blade having the function of a diaphragm blade by deformation of the electrostrictive element produced when a voltage is applied to the element, and is adapted to obtain a desired deformation quantity from the electrostrictive element regardless of the magnitude of an initial deformation quantity due to hysteresis of the element. A reference switch such as, for example, a photointerrupter operates when a predetermined reference deformation quantity is obtained in an area where the deformation quantity changes linearly in respect to variation of an applied voltage. A voltage obtained by superposing a voltage corresponding to a difference between a desired deformation quantity of the element and the reference deformation quantity on a voltage applied to the electrostrictive element when the reference switch is operated is applied to the electrostrictive element so that a desired deformation quantity is obtained regardless of the magnitude of the initial deformation quantity.