An Inverse Magnetron Injection Gun (IMIG) design is proposed for future conventional-type high-power gyrotrons. The design is deduced from the design for the KIT 2-MW, 170-GHz coaxial-cavity gyrotron prototype which has been published recently. As for the coaxial-cavity gyrotron IMIG, the inverse MIG design for the conventional hollow-type gyrotron is offering the possibility for compact geometry, optimum cooling of critical parts and excellent beam parameters due to its principle of construction. Considering the fundamental beam parameters, in the theoretical analysis an excellent beam quality has been achieved which results in a very low velocity spread.