A monochrometer mounted with the electron gun of an electron microscope or the like. This monochrometer does not need movement of a slit. An electron source consisting of any one of a thermal emission-type electron source (such as an LaB6 electron source or a tungsten hairpin), a Schottky emission-type electron source, and a tunneling field emission-type electron source is used. The slit is made of a single metal plate and mounted in position fixedly. Electrons are emitted from the electron source and dispersed within a plane including the slit according to energies. The slit is so positioned that it passes only those of the dispersed electrons which have energies close to the peak energy and blocks electrons having energies higher or lower than the peak energy.