The half-life of the ground state of Th229 (Th229g) has become an important factor in nuclear technology, for example, in the geological disposal of nuclear spent fuel. However, the values reported in two previous studies are not in agreement. This study reevaluates the half-life of Th229g by using a simple and reliable method. The U232/U233 activity ratio of a U232,233 sample was measured by high-resolution α-particle spectrometry. Next, the Th228/Th229g activity ratio of the Th sample, which was grown from the U232,233 sample, was also measured. The half-life of Th229g was calculated from these activity ratios, the growth time, and the half-lives of U232, U233, and Th228. From the results of these five measurements, the half-life of Th229g is determined to be 7932 ± 55 yr at a confidence level of 2σ.