A large incident angle of an off-axis principal ray is ensured while a stroke of a moving lens group is kept long, and a size is reduced while a focusing operation in magnification is easy.A magnifying endoscope optical system provided with a negative single lens cemented to an image pickup device and a moving lens group, capable of switching at least between a normal observation state and a proximity magnifying state can be made by moving the moving lens groups; and the following conditional expression (1) is satisfied: −65