A developing device capable of improving the image density and the dot reproducibility, and an image forming apparatus including the same are provided. A developing device applies an alternating voltage superimposed on a direct current voltage to a developing roller to thereby develop an electrostatic latent image formed on a photoreceptor with toner. A bias voltage waveform superimposed at this time has an original period (first period) in which each of a development-side electrical potential and an opposite development-side electrical potential is applied one time and a period (second period) in which a Vpp is gradually increased from the initial value to the maximum value.