A drop mass deviation measuring apparatus, a drop mass deviation measuring method, a pattern forming system, and a control method measure mass deviations of drops discharged from a plurality of drop discharge units in real time with high precision. The apparatus utilizes a plurality of drops discharged from a plurality of drop discharge units, a drop moving force providing part to provide moving forces, having directions different from discharge directions of each of the plurality of drops, to the plurality of drops, a discharged drop position detection member to acquire drop position images individually reflecting the a position of each of the plurality of drops, and a drop mass deviation measurement control part to calculate a drop discharge direction separation angle of each of the plurality of drops using the drop position images acquired by the discharged drop position detection member to measure mass deviation of each of the drops.