The design of a planar low-voltage cyclotron resonance maser (CRM) with the con-focal cavity, its geometry and results of the trajectory analysis for the planar magnetron - injection gun (MIG) are presented. Simulation results of dispersive characteristic of the confocal cavity and the distribution of an electromagnetic field for various cavity modes are described. The low-voltage CRM is designed for the frequency of 8 GHz with the water cooling magnetic system. Results of a computer simulation of wave particle interaction in the low-voltage CRM are also given.