1. A robot programming device for generating a palletizing program for use in a palletizing system including: a conveyor for conveying a plurality kinds of units, a vision sensor to recognize a kind of each unit conveyed by the conveyor based on the size thereof, a robot for assorting the units based on the kind of unit recognized by the vision sensor, and a plurality of pallets to which the assorted units are loaded according to the kinds of the units, the robot programming device comprising: a storing part for storing three-dimensional models of the plurality kinds of units, the conveyor, the vision sensor, the robot and the plurality of pallets; a layout making part for making a layout, in a virtual space, of the three-dimensional models stored in the storing part; a displaying part for displaying the layout made by the layout making part; an information setting part for setting information regarding each component displayed on the displaying part; and a program generating part for generating a palletizing program for the robot based on the layout of the three-dimensional models and the information set by the information setting part, wherein the palletizing program includes an algorithm for causing the vision sensor to output a result of recognition of each unit conveyed to a delivery position of the conveyor.