The present invention provides an image forming apparatus for forming an image by discharging liquid from a recording head onto a recording medium, comprising a carriage to which the recording head and a sub-tank for storing the liquid to be supplied to the recording head are mounted and which can be shifted reciprocally, a main tank disposed out of the carriage and adapted to store the liquid, a liquid flow passage connected between the main tank and the sub-tank in order to supply the liquid from the main tank to the sub-tank, a flow passage opening/closing valve disposed in the liquid flow passage on the way and adapted to open and close the liquid flow passage, and control means for controlling opening/closing of the flow passage opening/closing valve in accordance with an image forming condition during image formation using the recording head.