This disclosure concerns a speed detector sampling coordinates showing the position of a moving body and detecting the speed thereof includes a memory storing preset tentative angles and the values of a trigonometric function corresponding to the tentative angles; a first register storing the first coordinates of the moving body obtained by a first sampling; a second register storing the actual second coordinates of the moving body obtained by a second sampling subsequent to the first sampling; and an arithmetic operation unit calculating second coordinates in calculation, which are shown by multiplication, addition, or subtraction of the first coordinates and the values of the trigonometric function, to approximate the second coordinates in calculation to the actual second coordinates, the arithmetic operation unit calculating the speed of the moving body based on the tentative angle corresponding to the value of the trigonometric function used to the second coordinates in calculation.