To establish traceability of absorbed dose to water, a cobalt calibration coefficient is transferred to a reference ionization chamber by the standard dosimetry laboratory in the radiotherapy field. In Japan, the calibration is done against a set of an ionization chamber and an electrometer as system. Nowadays, solely electrometer calibration ^ desirable ^ measure absorbed dose witah. more than one combination of ionization chamber and electrometer. Unfortunately, there is no domestic electrometer calibration service for nano-ampere range appropriate for ionization curr e n measurement in the radiotherapy field. In this report, a relative electrometer calibration factor was determined by comparison between a calibrated combination and a non-calibrated electrometer under identical irradiation conditions at the user site. To estimate uncertainties of user electrometer calibration, comparison was made betw en calibration factor obtained by ionization current under a linac photon beam and DC current with a precision DC source instrument. It was found thatthe variation of electrometer readings to pulse ionization current is negligible under the steady photon beam output and dose monitor system. Therefore relative electrometer calibration under identical irradiation conditions at the user site was judged valid until a domestic nano-ampere electrometer calibration service becomes available.