PURPOSE: To vary precision characteristics of decision on a focusing state by providing a correcting means for optical path length which varies the length of an optical path of final split light and holding the direction of incidence to an array sensor almost the same.

CONSTITUTION: Image-forming luminous flux 21 transmitted through the transmission mirror part of a quick return mirror is split into three mutually parallel pieces of luminous flux by the half-mirror 9 of a light splitter 24 and projected on the array sensor 12, wherein a sensor 122 is placed on an expected image- formation surface and sensors 121 and 123 are at equal distance from it to perform normal focusing detection. When the light splitter 24 is moved slightly, luminous flux transmitted through a beam splitter 35 is reflected by a reflecting surface 36 of a prism to enter the sensor 123 with different optical path length. For this purpose, the output F3 of the sensor 123 beyond a focusing range 14 is viewed to increase greatly the range wherein a photographic lens is moved out and in, deciding on the focusing state.