An image forming apparatus comprising: an image bearing member configured to bear a developer image; a developer bearing member configured to bear developer; and a detection unit configured to detect a developer amount of a detection developer image formed on the image bearing member; wherein, in an image forming mode, the developer image is formed on the image bearing member by supply of developer from the developer bearing member to the image bearing member and in a detection mode, the detection developer image used for detection is formed on the image bearing member and a developer amount of the detection developer image is detected by the detection unit, characterized in that the apparatus is configured to, in a case where a peripheral speed ratio (v11/v12) between a peripheral speed of the developer bearing member and a peripheral speed of the image bearing member in the detection mode is denoted by Δv1, and a peripheral speed ratio (v21/v22) between a peripheral speed of the developer bearing member and a peripheral speed of the image bearing member in the image forming mode is denoted by Δv2, estimate a developer amount of the detection developer image in the image forming mode based on a result of a detection of a developer amount of the detection developer image in the detection mode in a state of Δv1 < Δv2 wherein (i) (Q/S) x Δv1 < C x ΔVc is satisfied in the detection mode and (ii) (Q/S) x Δv2 > C x ΔVc is satisfied in the image forming mode, where a capacitance of the image bearing member is denoted by C, a development contrast formed by a light portion potential of the image bearing member and a development potential of the developer bearing member is denoted by ΔVc, and a charge amount per unit area of developer borne by the developer bearing member is denoted by Q/S, wherein the image forming mode includes a first image forming mode and a second image forming mode, and wherein Δv3 < Δv2 is satisfied, where a peripheral speed of the developer bearing member in the first image forming mode is denoted by v21, a peripheral speed of the image bearing member is denoted by v22, a peripheral speed ratio (v21/v22) between the peripheral speed of the developer bearing member and the peripheral speed of the image bearing member is denoted by Δv2, and a peripheral speed ratio (v31/v32) between a peripheral speed of the developer bearing member and a peripheral speed of the image bearing member in the second image forming mode is denoted by Δv3. an image bearing member configured to bear a developer image; a developer bearing member configured to bear developer; and a detection unit configured to detect a developer amount of a detection developer image formed on the image bearing member; wherein, in an image forming mode, the developer image is formed on the image bearing member by supply of developer from the developer bearing member to the image bearing member and in a detection mode, the detection developer image used for detection is formed on the image bearing member and a developer amount of the detection developer image is detected by the detection unit, characterized in that the apparatus is configured to, in a case where a peripheral speed ratio (v11/v12) between a peripheral speed of the developer bearing member and a peripheral speed of the image bearing member in the detection mode is denoted by Δv1, and a peripheral speed ratio (v21/v22) between a peripheral speed of the developer bearing member and a peripheral speed of the image bearing member in the image forming mode is denoted by Δv2, estimate a developer amount of the detection developer image in the image forming mode based on a result of a detection of a developer amount of the detection developer image in the detection mode in a state of Δv1 < Δv2 wherein (i) (Q/S) x Δv1 < C x ΔVc is satisfied in the detection mode and (ii) (Q/S) x Δv2 > C x ΔVc is satisfied in the image forming mode, where a capacitance of the image bearing member is denoted by C, a development contrast formed by a light portion potential of the image bearing member and a development potential of the developer bearing member is denoted by ΔVc, and a charge amount per unit area of developer borne by the developer bearing member is denoted by Q/S, wherein the image forming mode includes a first image forming mode and a second image forming mode, and wherein Δv3 < Δv2 is satisfied, where a peripheral speed of the developer bearing member in the first image forming mode is denoted by v21, a peripheral speed of the image bearing member is denoted by v22, a peripheral speed ratio (v21/v22) between the peripheral speed of the developer bearing member and the peripheral speed of the image bearing member is denoted by Δv2, and a peripheral speed ratio (v31/v32) between a peripheral speed of the developer bearing member and a peripheral speed of the image bearing member in the second image forming mode is denoted by Δv3.