1. An occupant information detection system for detecting information related to a vehicle occupant on a vehicle seat, comprising: an image detecting device configured to face a vehicle seat area and obtain a three-dimensional image; an information extraction processing section of a computing device configured to extract information related to a three-dimensional surface profile of a detected object on a vehicle seat based on the three-dimensional image; a conversion processing section of the computing device configured to convert the extracted information into numerical coordinates of a predetermined coordinate system; a computation processing section of the computing device configured to derive a computed value by subtracting an average of first and second coordinate components from a third coordinate component, wherein the first and second coordinate components are arbitrary points and the third coordinate component is a middle point of the arbitrary points on a coordinate axis extending between the detected object and the image detecting section in the numerical coordinates; and a determination processing section of the computing device determining the concave-convex shape of a line segment formed by the arbitrary points and the middle point based on the computed value; wherein the determination processing section is configured to determine that the line segment forms a convex shape as seen from the image detecting section when the computed value is negative, that the line segment forms a concave shape as seen from the image detecting section when the computed value is positive, and that the line segment forms a plane when the computed value is zero.