Essential increase of operation efficiency of electron cyclotron resonance (ECR) multi-charged ion (MCI) sources (that is the increase of the average charge number and current of ion beams) is connected with possibility to achieve more dense plasma and to achieve so called quasi-gasdynamic regime of the plasma confinement in mirror traps using RF radiation with higher frequency. Results of experimental investigation of ion charge state distribution in a pulsed ECR source of MCI with pumping by millimeter wave radiation of powerful gyrotron are presented in the paper. Millimeter wave radiation with maximum power W=130 kW, frequency f=37.5 GHz and pulse duration up to 1.5 ms is used in the experiments. The charge state distribution maximum corresponded to argon charge (+13)-(+15). Total ion saturation current density of the probe achieved 2 A/cm2.