An image forming device is provided with: a zoom lens system TL that performs magnification varying by changing the distance between the lens units; and an image sensor SR that converts an optical image formed by the zoom lens system TL into an electric signal. The zoom lens system TL includes at least a negative first lens unit GR1 and a positive second lens unit GR2, and at least the first lens unit GR1 and the second lens unit GR2 move in zooming from the wide angle end (W) to the telephoto end (T). The second lens unit GR2 includes three lens elements of, from the object side, a positive meniscus lens element L21 convex to the object side, a negative lens element L22 and a positive lens element L23, and the following condition is satisfied: -1.0<(CR1-CR2)/(CR1+CR2)<0.0 (CR1 is the front radius of curvature of the most object side lens element L21 in the second lens unit GR2, and CR2 is the rear radius of curvature of the most object side lens element L21 in the second lens unit GR2).