To provide an inexpensive microscope objective lens of an achromat class which has various aberrations compensated excellently up to an image periphery by providing specific constitution and meeting specific conditions.

This lens is equipped with a 1st lens group G1 which consists of a cemented lens of a biconcave lens L1n and a biconvex lens L1p, a 2nd lens group G2 consisting of a positive lens, and a 3rd lens group G3 consisting of a cemented lens of a biconcave lens L3n and a biconvex lens L3p. Then the lens meets the conditions of -1.0≤f/f1≤1.0, 0<n3n3-n3p, and 20≤ν1p-pm1n. Here, (f) is the focal length of the whole lens system, f1 the focal length of the 1st lens group G1, and n3p and n3n are the refractive indexes of the biconvex lens L3p and biconcave lens L3n to a (d) line. Then ν1p and ν1n are the Abbe numbers of the biconvex lens L1p and biconcave lens L1n of the 1st lens group G1.