While maintaining a longer stroke of the moving lens group, and to ensure the angle of incidence of a large off-axis principal ray is small size yet easy focusing operation at the time of zooming. A plurality of lens groups, and a negative lens joined to the image sensor is capable of switching between at least a normal observation state and the near-expanded state by moving some of lens groups from among the lens groups, the following to provide expanded endoscope optical system satisfying the conditional expression (1). -65
