Disclosed is a microtron electron accelerator having an accelerating cavity accepting microwave electric power for generating a high-frequency accelerating electric field E disposed within a uniform magnetic field B and adapted such that electrons are accelerated and caused to move in a circular trajectory under action of the magnetic field B and the electric field E, comprising an electron source formed of a cathode and an anode, which has a minute slit allowing an electron beam extracted from the cathode to pass therethrough, disposed on the outer side of the wall of the accelerating cavity, a first electron beam through-hole and a second electron beam through-hole formed in the wall of the accelerating cavity in two positions, with the electron source therebetween, along the decreasing or increasing direction of the strength of the electric field E in the accelerating cavity, and a third electron beam through-hole formed in the wall of the accelerating cavity in a position in confrontation with the first electron beam through-hole across the inner space of the accelerating cavity. By adopting the above described structure, it has been made possible to have the energy gain within the accelerating cavity at each time of acceleration increased and to have contamination of the inner surface of the accelerating cavity by evaporated cathode material decreased, and as a result, it is made possible to obtain a microtron electron accelerator smaller in size and capable of stably providing a high-energy electron beam.