An image forming apparatus includes a head tank and a main tank that sends liquid to the head tank. When the remaining ink amount in the main tank is less than or equal to a predetermined amount (near empty), the liquid sending amount to be sent from the main tank to the head tank required for detecting a full state of the head tank is set to be a second liquid sending amount which is greater than a usual first liquid sending amount. An ink supply operation is performed to send liquid corresponding to the second liquid sending amount, from the main tank to the head tank. Then, it is determined whether the head tank is in a full state. When the head tank is not in a full state, the main tank is determined to be in an ink empty state.