A robot system, characterized by: a first conveyor configured to convey a plurality of kinds of workpieces; a second conveyor configured to convey workpiece groups each comprising a combination of the plurality of kinds of workpieces; a robot; and a controller configured to control the robot, the controller comprising: a situation acquirer configured to acquire a situation in a predetermined monitor area on the first conveyor and the second conveyor, the predetermined monitor area being set based on a movable range of the robot; a determiner configured to, based on the situation acquired by the situation acquirer, determine whether at least one workpiece among the plurality of kinds of workpieces on the first conveyor in the predetermined monitor area is a missing workpiece of at least one workpiece group among the workpiece groups in the predetermined monitor area; and an instructor configured to instruct the robot to hold the at least one workpiece determined as the missing workpiece by the determiner and to transfer the at least one workpiece from the first conveyor to the second conveyor. characterized in that the at least one workpiece determined as the missing workpiece by the determiner comprises a plurality of missing workpieces, wherein the at least one workpiece group among the workpiece groups in the predetermined monitor area is positioned in a most downstream area of the second conveyor in the predetermined monitor area, and wherein the instructor is configured to instruct the robot to prioritize one missing workpiece, among the plurality of missing workpieces, that is the missing workpiece of the at least one workpiece group so as to hold the one missing workpiece and transfer the one missing workpiece from the first conveyor to the second conveyor. a first conveyor configured to convey a plurality of kinds of workpieces; a second conveyor configured to convey workpiece groups each comprising a combination of the plurality of kinds of workpieces; a robot; and a controller configured to control the robot, the controller comprising: a situation acquirer configured to acquire a situation in a predetermined monitor area on the first conveyor and the second conveyor, the predetermined monitor area being set based on a movable range of the robot; a determiner configured to, based on the situation acquired by the situation acquirer, determine whether at least one workpiece among the plurality of kinds of workpieces on the first conveyor in the predetermined monitor area is a missing workpiece of at least one workpiece group among the workpiece groups in the predetermined monitor area; and an instructor configured to instruct the robot to hold the at least one workpiece determined as the missing workpiece by the determiner and to transfer the at least one workpiece from the first conveyor to the second conveyor. a situation acquirer configured to acquire a situation in a predetermined monitor area on the first conveyor and the second conveyor, the predetermined monitor area being set based on a movable range of the robot; a determiner configured to, based on the situation acquired by the situation acquirer, determine whether at least one workpiece among the plurality of kinds of workpieces on the first conveyor in the predetermined monitor area is a missing workpiece of at least one workpiece group among the workpiece groups in the predetermined monitor area; and an instructor configured to instruct the robot to hold the at least one workpiece determined as the missing workpiece by the determiner and to transfer the at least one workpiece from the first conveyor to the second conveyor. characterized in that the at least one workpiece determined as the missing workpiece by the determiner comprises a plurality of missing workpieces, wherein the at least one workpiece group among the workpiece groups in the predetermined monitor area is positioned in a most downstream area of the second conveyor in the predetermined monitor area, and wherein the instructor is configured to instruct the robot to prioritize one missing workpiece, among the plurality of missing workpieces, that is the missing workpiece of the at least one workpiece group so as to hold the one missing workpiece and transfer the one missing workpiece from the first conveyor to the second conveyor.