Transverse ion and neutral temperatures at the electron cyclotron resonance position were measured in an electron cyclotron resonance (ECR) plasma using a high-resolution optical emission spectroscopy of Doppler profiles of Ar and Ar+ transition. The transverse ion temperature increase from approximately 0.4 to 1.0 eV as the operating gas pressure is lowered from 1.0 to 0.1 mTorr. On the other hand, the transverse neutral temperature is much lower than the transverse ion temperature. By means of Langmuir probe measurement, large-amplitude potential fluctuation was observed at the ECR position under the low gas pressure. These observations suggest that the large-amplitude potential fluctuation directly affects the transverse ion temperature.