The 222Rn concentrations in indoor workplaces were measured in Aomori Prefecture, Japan, and the results are reported here. This survey was part of a program to measure background natural radiation dose rate in the prefecture where the first Japanese nuclear fuel cycling facilities are now under construction. The survey of the 222Rn concentrations in indoor workplaces was carried out at 107 locations from 1996 to 1998. The 222Rn concentrations were measured for approximately one year at each site with passive Rn detectors, which used a polycarbonate film for counting α-rays and could separate concentrations of 222Rn from 220Rn. Weeklong measurements of 222Rn concentration and working level were carried out with active detectors to get the ratio of 222Rn concentration during working hours to non-working hours as well as equilibrium factors in selected locations. Diurnal variation of 222Rn concentration depended on building structure, air-conditioning, time of day and day of the week (week days or weekend). The 222Rn concentration during working hours was generally lower than that in non-working hours. Although the annual average 222Rn concentration in indoor occupational environments was higher than that in dwellings, radiation dose for Aomori Prefecture residents from Rn in the former was 14% of the total indoor dose by Rn because of the lower concentration in working hours and lower occupancy factor.