To provide an optical system for a magnifying endoscope having a large magnification while contriving shortening of entire length and a wide viewing angle to the same extent as a single focus endoscope for screening.

The optical system, where the best focus distance to an object is varied, satisfies following conditions (1) and (2). (1) 2p/(1.22)FT4p/(1.22), (2) FWFT, wherein FT is an effective f-number when close-range observation, FW is an effective f-number when long-distance observation, p is a pixel pitch of CCD and is the wavelength (587 nm) of a d-line.