Disclosed is a focus detection apparatus including a light source; focusing means for focusing a light beam emitted from the light source onto an object; a prism for reflecting the light beam from the light source by its surface and for allowing return light from the object to pass therethrough to cause an astigmatism; and detection means for detecting a focusing state of the light beam on the object from the astigmatism of the return light. In this apparatus, the detection means is arranged at a position opposite to the light source to sandwich the prism therebetween, and the return light emerges from a surface thereof different from the incident surface to be guided to the detection means.