A groundwater dating for very old porewater using 36Cl and 4He was applied to the Koetoi andWakkanai formations distributed in the northernmost part in Japan.Measured 36Cl/Cl in theKoetoi Formationwas 2.6 ± 2.0 × 10-15 and that in theWakkanai Formation was 8.1 × 2.5 × 10-15.These values are similar to 36Cl/Cl in situ secular equilibrium calculated from chemical compositions of core suggesting that Cl- ions and porewater have remained in the formations formuch longer than half-life of 36Cl. He concentration in porewater ranged from 1.1 × 10-6 to 2.6 × 10-5 (ccSTP gw -1) and it is much higher than water saturated with air indicating that both formations contain very old porewater. However, the possibility of mixing of young water was indicated because He concentration was lower than that calculated bymultiplication of in situHe production and time after the uplift.This possibilitywas also supported by Cl-, δD, and δ 18O data. After combining information on 36Cl/Cl, 4He, and δD and δ18O, it was inferred that the porewater in the deep part of the Wakkanai Formation might have been stagnant since the uplift.The porewater in the Koetoi Formation and the shallow part of theWakkanai Formation were found to be affected by young surface water.