1. A support pin arrangement determination assisting method operatively associated with an already mounted surface of a board on a mounting stage provided with a board holding unit of an electronic component mounting machine, said support pin arrangement determination assisting method comprising steps of:displaying an image for pin arrangement determination assistance including an image of the board sticking out length or portion from the mounting stage, the image of the board indicating a shape and an arrangement of an already mounted component on the already mounted surface of the board;
inputting an arrangement position of a support pin to the displayed image of the board in a first mounting work position on the mounting stage to generate a first pin arrangement image;
superimposing the first pin arrangement image indicating the input arrangement position on the image of the board in the first mounting work position to create a first composite image;
displaying the first composite image;
generating a second pin arrangement image on the basis of the first pin arrangement image with an assumption that the board is positioned at a second mounting work position downstream from the first mounting work position where at least a part of an area of the board that was not on the mounting stage at the first mounting work position is on the mounting stage;
superimposing the first pin arrangement image and the second pin arrangement image on the board image to create a second composite image; and
displaying the second composite image.