Disclosed is an apparatus adapted to be used in a single lens reflex camera for detecting whether a correct focal adjustment has been made. The apparatus includes a pair of focal point detecting optical systems placed relative to each other on symmetrical positions with respect to an extending optical axis which is turned by an optical reflecting means, at the rear of a semitransparent movable mirror. A position where an image of an object on a main optical axis of the object lens, is focused by said object lens when an observing point of the object is placed in a position with an infinity distance, is determined as a basic point. The optical axes of said pair of focal point detecting optical systems cross with each other at said basic point. A photo-electric detecting plate is arranged in the vicinity of a film surface. A position on said detecting plate where an image of the object on said basic point is projected by said focal point detecting optical systems is determined as a cardinal point. A correct focusing is detected electrically when the observing point of the object is projected onto said cardinal point.