An image forming apparatus, comprising: a developing apparatus for developing an electrostatic latent image formed on an image bearing member with a developer, the developing apparatus including a developer carrying member for carrying the developer to develop the electrostatic latent image with the developer and a developer supplying member provided in contact with the developer carrying member for supplying the developer to the developer carrying member, the developer carrying member having a core electrode for rotatably supporting the developer carrying member, and the developer supplying member having a surface foam layer in which the developer can enter and a core electrode for rotatably supporting the developer supplying member; and developing apparatus moving means for moving the developing apparatus between a first position in which the developer carrying member is in contact with the image bearing member to perform a developing operation and a second position in which the developer carrying member is separated from the image bearing member not to perform the developing operation; characterized by comprising detecting means configured to detect a capacitance between the core electrode of the developer carrying member and the core electrode of the developer supplying member, wherein the detecting means is connected to the developer supplying member and disposed to detect a voltage induced to the developer supplying member when applying an alternating bias to the core electrode of the developer carrying member or the detecting means is connected to the developer carrying member and disposed to detect a voltage induced to the developer carrying member when applying an alternating bias to the core electrode of the developer supplying member, wherein the detecting means is configured to detect the capacitance when a rotation of the developer supplying member is stopped at a time when the developing apparatus is in the second position. a developing apparatus for developing an electrostatic latent image formed on an image bearing member with a developer, the developing apparatus including a developer carrying member for carrying the developer to develop the electrostatic latent image with the developer and a developer supplying member provided in contact with the developer carrying member for supplying the developer to the developer carrying member, the developer carrying member having a core electrode for rotatably supporting the developer carrying member, and the developer supplying member having a surface foam layer in which the developer can enter and a core electrode for rotatably supporting the developer supplying member; and developing apparatus moving means for moving the developing apparatus between a first position in which the developer carrying member is in contact with the image bearing member to perform a developing operation and a second position in which the developer carrying member is separated from the image bearing member not to perform the developing operation; characterized by comprising detecting means configured to detect a capacitance between the core electrode of the developer carrying member and the core electrode of the developer supplying member, wherein the detecting means is connected to the developer supplying member and disposed to detect a voltage induced to the developer supplying member when applying an alternating bias to the core electrode of the developer carrying member or the detecting means is connected to the developer carrying member and disposed to detect a voltage induced to the developer carrying member when applying an alternating bias to the core electrode of the developer supplying member, wherein the detecting means is configured to detect the capacitance when a rotation of the developer supplying member is stopped at a time when the developing apparatus is in the second position.