There is provided a position detecting apparatus that has a high ability of detecting a correct position even if there is mechanical play in a moving mechanical member, a position of which is detected, and an image processing apparatus provided with the position detecting apparatus. In a position detecting apparatus 100 of the invention, even if there is a certain degree of play in a slide plate 11 that moves in a slide direction DN and reading by bit mark reading sensors 21 to 24 deviates from regular reading of binary numbers, since binary numbers appended to a surface of the slide plate is formed by gray codes, a position derived by a position deriving unit is more likely to be controlled to be as small as adjacent binary numbers with respect to a slide direction of the slide plate compared with the case of the conventional simple binary numbers.