An image forming device includes a casing and a medium detecting mechanism. A medium entrance is formed on the casing. The medium detecting mechanism includes a detector, a first pivoting component, a second pivoting component and a resilient component. When at least one medium enters into the medium entrance and abuts against the first pivoting component, the at least one medium drives the first pivoting component to pivot in a first pivoting direction, so that the second pivoting component is driven by the resilient component to pivot in the first pivoting direction to actuate the detector for generating a signal. When the second pivoting component is stopped from pivoting in the first pivoting direction, deformation of the resilient component allows the first pivoting component to be driven by the at least one medium to pivot relative to the second pivoting component in the first pivoting direction continuously.