An optical type granule sorting machine is provided which allows a sensitivity setting to be easily performed by effectively utilizing RGB three-dimensional color space information similar to information obtained via human eyes and which enables signal processing to be substantially simplified. Determination means includes a three-dimensional color distribution data creation section that creates data on wavelength components of R light, G light, and B light from the granules, on a three-dimensional color space, a Mahalanobis distance interface creation section that sets an interface calculated based on a Mahalanobis distance to partition the data into a conforming-granule cluster area and a nonconforming granule cluster area, a Euclidean distance interface creation section that determines a position of center of gravity of the conforming granule cluster area and a position of center of gravity of the nonconforming granule cluster area to set an interface calculated based on a Euclidean distance at which the positions of center of gravity lie at a longest distance from each other, and a threshold determination section that determines a line of intersection between the interface calculated based on the Mahalanobis distance and the interface calculated based on the Euclidean distance, to determine the line of intersection to be a determination threshold that allows determination of whether or not the granules are to be treated as a separation target.