Because mixing of groundwater commonly occurs in geologically active regions such as subduction zones, sole use of a conventional method for groundwater dating is not applicable. Here we present a new model using dissolved 4He concentration and He isotopic ratio as well as crustal He flux to estimate the very long residence time of groundwater where mixing of waters of different origin occurs. We applied this model to the Kobe area, southwest Japan, where Arima-type thermal brine with a high 3He/4He (1 ¿ 10-5) component is incorporated into meteoric water. A groundwater residence time of 25--230 kyr is estimated for the deep groundwater (up to 1500 m depth) aquifers. The flux of Arima-type brine is also estimated to be 2 ¿ 105 m3 yr-1 in the studied area (20 km ¿ 5 km). These attempts contribute to evaluating the long-term stability of geological formations over the time range of 104--105 years and to considering the deep-seated fluid flux to shallower aquifers.