The atmospheric electric field was measured with a field mill at Sanriku during October 1975-October 1976. Using data thus obtained of 150 undisturbed days during the period, the annually averaged diurnal variation of electric field was deduced. It showed the UT variation with a small local-time effect. The phase of the local-time variation is shifted behind by about 2hrs from the normal local-time variation. The main sources of condensation nuclei which cause the local-time variation at Sanriku are deduced to be a train of large cities located inland about 50-90km southwest to northwest of Sanriku.