A jig design assisting device and a robot simulation device that can create a rough virtual model of a jig through a simple operation and assist a designer. The assisting device includes: a display section; a storage section that stores three-dimensional models of a workpiece and a jig; a workpiece designating section that designates the type of the workpiece; a workpiece locating section that automatically locates the designated workpiece in a three-dimensional space; a block selecting section that selects the type and location method of the positioning block; a clamper selecting section that selects the type of the clamper; a model part designating section that designates a part of the workpiece model; a jig locating section that locates the positioning block or the clamper within a three-dimensional space; and an adjusting section that automatically adjusts the dimension or shape of the three-dimensional model of the positioning block or the clamper.