The effects of an electric field shield on the electric field noise and neutralizing current of a corona discharge air ionizer were investigated as a means to prevent soft errors during quality control inspections. The neutralizing current and noise voltage were measured for shielded and nonshielded air ionizers using a charged plate monitor. The use of an electric field shield decreased both the neutralizing current and the noise voltage. Therefore, neutralizing current to noise voltage ratio was calculated to evaluate the effect of the shield on the neutralization performance, and this ratio was several times higher for the shielded air ionizer than for the nonshielded air ionizer. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.