1. A symmetrical field type objective lens of an electron microscope which is of a rotationally symmetric configuration relative to an optical axis of a magnetic field type electron lens system and comprises an upper magnetic pole piece and a lower magnetic pole piece disposed with a predetermined distance therebetween, and bores of a same diameter which are formed in said upper and lower magnetic pole pieces, respectively, wherein said predetermined distance represented by S and said diameter of said bores represented by b are so selected that a relation given by the following expression EQU 1.ltoreq.S/b.ltoreq.5 is fulfilled, and wherein magnetic excitation for said objective lens represented by J is so selected with respect to a magnetomotive force of Riecke-Ruska's condenser objective lens represented by Jc.o. at which an electron beam incident on said objective lens in parallel with said optical axis once intersects said optical axis and leaves said objective lens in parallel with said optical axis that for a given value of S/b in the range defined above, the following condition EQU 1.4 Jc.o..ltoreq.J.ltoreq.1.7 Jc.o. is fulfilled.