The isotope ²¹⁰Po was suspected of being involved in the death of a former Russian intelligence agent in 2006 in the UK. Although human exposure to this natural radionuclide in foods is estimated to be high, few studies are available. UNSCEAR Report 2000 does not contain data on ²¹⁰Po concentrations of foodstuffs in Japan. We analyzed samples of the everyday Japanese diet cooked with foodstuffs purchased at supermarkets in 7 major domestic cities in 2007-2008. ²¹⁰Po was quantified by alpha spectrometry and natural radionuclides such as ⁴⁰K by gamma spectrometry. The daily intake and committed effective dose of ²¹⁰Po, ⁴⁰K, and other natural radionuclides for Japanese adults were calculated. Daily intake was 0.34-1.84 (mean ± σ : 0.66 ± 0.53) and 68.5-94.2 (81.5 ± 8.5) Bq/d and the committed effective dose was 0.15-0.81 (0.29 ± 0.24) and 0.16-0.21 (0.18 ± 0.02) mSv for ²¹⁰Po and ⁴⁰K, respectively, comprising a high percentage of the total exposure. The total of the mean committed effective dose for the two nuclides (0.47 mSv) was higher than the annual effective dose from ingestion of foods reported by UNSCEAR 2000 (0.29 mSv). The mean committed effective dose of ⁴⁰K in the 7 major Japanese cities was comparable to the global average (0.17 mSv). The dietary exposure of Japanese adults can be characterized by a higher ²¹⁰Po contribution than in other countries. Of the total daily dietary ²¹⁰Po exposure (13 food categories excluding water) for adults in Yokohama, about 70% was from fish/shellfish and 20% from vegetables/mushrooms/seaweeds, reflecting preferences of Japanese to eat a considerable amount of fish/shellfish containing high ²¹⁰Po concentrations.