An image forming apparatus comprising: a recording head configured to discharge liquid droplets; a sub-tank configured to contain liquid to be supplied to the recording head; a carriage configured to carry the recording head and the sub-tank; a main tank configured to contain the liquid to be supplied to the sub-tank; a liquid feeding part configured to supply the liquid from the main tank to the sub-tank, a displacement member provided in the sub-tank and configured to change its position according to a remaining amount of the liquid in the sub-tank so that the remaining amount of the liquid in the sub-tank when the displacement member is at a first position is smaller than the remaining amount of the liquid in the sub-tank when the displacement member is at a second position, a second detection part provided on a body of the image forming apparatus and configured to detect whether the displacement member is at the predetermined second position, characterized by: a first detection part provided on the carriage and configured to detect whether the displacement member is at the predetermined first position, control means for detecting and storing a differential supply amount corresponding to a displacement amount of the displacement member between the position at which the displacement member is detected by the first detection part and the position at which the displacement member is detected by the second detection part and to control the liquid feeding part so that when the liquid is supplied to the sub-tank from the main tank during image forming, when the carriage is moving without using the second detection part, the liquid is supplied to the sub-tank until the first detection part detects the displacement member and further in the different supply amount, wherein when the liquid is to be supplied to the sub-tank fom the main tank without using the second detection part, the liquid is supplied from the main tank to the sub-tank when a liquid consumption amount after the first detection part has detected the displacement member becomes a predetermined liquid consumption amount. a recording head configured to discharge liquid droplets; a sub-tank configured to contain liquid to be supplied to the recording head; a carriage configured to carry the recording head and the sub-tank; a main tank configured to contain the liquid to be supplied to the sub-tank; a liquid feeding part configured to supply the liquid from the main tank to the sub-tank, a displacement member provided in the sub-tank and configured to change its position according to a remaining amount of the liquid in the sub-tank so that the remaining amount of the liquid in the sub-tank when the displacement member is at a first position is smaller than the remaining amount of the liquid in the sub-tank when the displacement member is at a second position, a second detection part provided on a body of the image forming apparatus and configured to detect whether the displacement member is at the predetermined second position, characterized by: a first detection part provided on the carriage and configured to detect whether the displacement member is at the predetermined first position, control means for detecting and storing a differential supply amount corresponding to a displacement amount of the displacement member between the position at which the displacement member is detected by the first detection part and the position at which the displacement member is detected by the second detection part and to control the liquid feeding part so that when the liquid is supplied to the sub-tank from the main tank during image forming, when the carriage is moving without using the second detection part, the liquid is supplied to the sub-tank until the first detection part detects the displacement member and further in the different supply amount, wherein when the liquid is to be supplied to the sub-tank fom the main tank without using the second detection part, the liquid is supplied from the main tank to the sub-tank when a liquid consumption amount after the first detection part has detected the displacement member becomes a predetermined liquid consumption amount. a first detection part provided on the carriage and configured to detect whether the displacement member is at the predetermined first position, control means for detecting and storing a differential supply amount corresponding to a displacement amount of the displacement member between the position at which the displacement member is detected by the first detection part and the position at which the displacement member is detected by the second detection part and to control the liquid feeding part so that when the liquid is supplied to the sub-tank from the main tank during image forming, when the carriage is moving without using the second detection part, the liquid is supplied to the sub-tank until the first detection part detects the displacement member and further in the different supply amount, wherein when the liquid is to be supplied to the sub-tank fom the main tank without using the second detection part, the liquid is supplied from the main tank to the sub-tank when a liquid consumption amount after the first detection part has detected the displacement member becomes a predetermined liquid consumption amount.