The possibility of using an Electron Cyclotron Resonance (ECR) ionizer for polarized 3 He gas as a compact polarized 3 He ion source is investigated by the Monte Carlo simulation over an ECR frequency range from 2 to 16 GHz . In this calculation, we assume that the depolarization mechanism is caused by multiple ionization, inelastic scattering, and de-ionization processes of the 3 He atoms and ions confined in the ECR plasma under the presence of the external magnetic field and the microwave for ECR. To check the validity of the present Monte Carlo simulation, we calculate the proton polarization following the ionization of polarized hydrogen gas by the ECR ionizer and find consistency between the calculated results and experimental ones. With a typical value of the 3 He gas polarization currently available, i.e. P gas∼0.5, a polarized 3 He 2+ beam with the maximum polarization less than 0.2 can be expected from the polarized 3 He ion source based on the ECR ionizer.