The plasma potential of liquid-He-free superconducting electron cyclotron resonance ion source was measured as a function of minimum strength of mirror magnetic field (B(min)) and gas pressure with the method based on the retarding electric field. We observed that the plasma potential decreased with increasing B(min) up to 0.5 T and then gradually increased again. The plasma potential increased with increasing gas pressure. When we add the O(2) gas to the Ar plasma (gas mixing method), plasma potential gradually decreased with increasing the O(2) gas pressure.