An image forming apparatus comprising: an apparatus main body; a recording head configured to discharge liquid droplets; a sub tank configured to contain liquid to be supplied to the recording head; a carriage on which the recording head and the sub tank are mounted; a main tank configured to contain liquid to be supplied to the sub tank; a fluid feeding unit configured to supply the liquid from the main tank to the sub tank, a displacement member on the sub tank and configured to be positioned depending on a remaining amount in the sub tank, so that the remaining amount at a predetermined first position is less than the remaining amount at a predetermined second position, a second detecting unit on the apparatus main body and configured to detect whether the displacement member is at the predetermined second position, characterized by: a first detecting unit on the carriage and configured to detect whether the displacement member is at the predetermined first position, a control section configured to detect and store a differential supply amount of the liquid corresponding to a displacement amount of the displacement member, the displacement amount corresponding to a difference between a position detected by the first detecting unit and a position detected by the second detecting unit, and to control the fluid feeding unit to supply the differential supply amount of the liquid to the sub tank after the first detecting unit detects the displacement member in a case where liquid is supplied from the main tank to the sub tank without using the second detecting unit during image forming, when the carriage 33 is moving. an apparatus main body; a recording head configured to discharge liquid droplets; a sub tank configured to contain liquid to be supplied to the recording head; a carriage on which the recording head and the sub tank are mounted; a main tank configured to contain liquid to be supplied to the sub tank; a fluid feeding unit configured to supply the liquid from the main tank to the sub tank, a displacement member on the sub tank and configured to be positioned depending on a remaining amount in the sub tank, so that the remaining amount at a predetermined first position is less than the remaining amount at a predetermined second position, a second detecting unit on the apparatus main body and configured to detect whether the displacement member is at the predetermined second position, characterized by: a first detecting unit on the carriage and configured to detect whether the displacement member is at the predetermined first position, a control section configured to detect and store a differential supply amount of the liquid corresponding to a displacement amount of the displacement member, the displacement amount corresponding to a difference between a position detected by the first detecting unit and a position detected by the second detecting unit, and to control the fluid feeding unit to supply the differential supply amount of the liquid to the sub tank after the first detecting unit detects the displacement member in a case where liquid is supplied from the main tank to the sub tank without using the second detecting unit during image forming, when the carriage 33 is moving. a first detecting unit on the carriage and configured to detect whether the displacement member is at the predetermined first position, a control section configured to detect and store a differential supply amount of the liquid corresponding to a displacement amount of the displacement member, the displacement amount corresponding to a difference between a position detected by the first detecting unit and a position detected by the second detecting unit, and to control the fluid feeding unit to supply the differential supply amount of the liquid to the sub tank after the first detecting unit detects the displacement member in a case where liquid is supplied from the main tank to the sub tank without using the second detecting unit during image forming, when the carriage 33 is moving.