An image forming apparatus includes a first image bearing member; a first charger for electrically charging the first image bearing member; a first developer carrying member for carrying a first color developer to develop an electrostatic image formed on the first image bearing member with the first color developer; a first voltage source for applying a first oscillating voltage to the first developer carrying member; a second image bearing member; a second charger for electrically charging the second image bearing member; a second developer carrying member for carrying a second color developer to develop an electrostatic image formed on the second image bearing member with the second color developer; a second voltage source for applying a second oscillating voltage to the second developer carrying member; and a third voltage source for applying a common DC voltage to the first charger and to the second charger, wherein a frequency of the first oscillating voltage and a frequency of the second oscillating voltage are substantially the same.