A developing device and an image forming apparatus are provided, which prevent the output of the sensor from being destabilized due to the loose aggregation and stagnation of a developer on the detection surface of a permeability sensor and stabilize the output of the permeability sensor so as to enable highly precise toner concentration control. The developing device includes a transport passage, a developing roller that supplies to a photosensitive drum the two-component developer passing through the transport passage, a transport member that stirs the two-component developer in the transport passage and supplies the two-component developer to the developing roller, and the permeability sensor that is disposed in the transport passage to detect a toner concentration of the two-component developer. The developing device also includes an electromagnet located adjacent to the permeability sensor. The electromagnet is supplied with an intermittent flow of current to make the developer active in movement.