An optical system for rigid endoscope includes an objective optical system, an eyepiece optical system, and a relay optical system which is disposed between the objective optical system and the eyepiece optical system. The objective optical system includes in order from an object side, a first lens having a negative refractive power, a second lens having a positive refractive power, a third lens having a positive refractive power, and a fourth lens having a negative refractive power. The following conditional expressions (1) and (2) are satisfied:−；3<；(RL1i+RL1o)/(RL1i−；RL1o)<；−；1.3 ； ；(1)30<；vdL1 ； ；(2)where,RL1o denotes a radius of curvature of an object-side surface of the first lens,RL1i denotes a radius of curvature of an image-side surface of the first lens, andvdL1 denotes Abbe number for the first lens.
