1. An endoscope inserting direction detecting method comprising: a first step of determining an inserting-direction candidate, that is, a candidate for an endoscope inserting direction in which an endoscope should be inserted; a second step of receiving an endoscopic image; a third step of setting sampling pixels out of all pixels on the endoscopic image; a fourth step of detecting the direction of a change in brightness in the endoscopic image on a basis of each sampling pixel set in the third step; a fifth step of detecting the direction of a charge in brightness from N sampling pixels (where N denotes an integer), expressing the direction of a change in brightness in the form of a first vector Vn (where 1≦n≦N), expressing K candidates for the endoscope inserting direction (where K denotes an integer) in the form of a second vector Uk (where 1≦k≦K) originating from the locations of the sampling pixels set in the third step, and evaluating an angle between the first vector Vn and the second vector Uk; a sixth step of evaluating the angles among a plurality of inserting-direction candidates and the direction of the change in brightness on the basis of a result of the evaluation in the fifth step; and a seventh step of determining an endoscope inserting direction, in which an endoscope should be inserted, on the basis of a result of the evaluation in the sixth step.