A multi-beam scan optical system and an image forming apparatus using the same are provided in which high-quality printing can be realized at high speed with a relatively simple construction. In a magnetoresistance effect element wherein a sense current is caused to flow in a direction perpendicular to the plane of the film, a resistance regulating layer is provided in at least one of a pinned layer, a free layer and an non-magnetic intermediate layer. When it is desired to pass the fluids to the sterile field, the enclosure is separated into at least two parts to allow the syringe to be removed.