Gas entrainment (GE) phenomena, such as a pump sump, are observed in many industrial scenes. Moreover, in design studies of Japanese sodium-cooled fast reactors, occurrences of GEs are a concern because a compact reactor vessel is expected to be employed owing to its economical advantages and coolant velocity being higher than that of conventionally designed vessels. However, the occurrence conditions of GEs are too difficult to predict by theoretical methods because they are usually highly nonlinear phenomena. Therefore, we are developing a CFD method to evaluate the GE accurately. In this paper, formulations of high-precision volume-of-fluid algorithm are presented. Since accurate geometric modeling of complicated reactor systems is a key to simulate GEs, nonorthogonal meshes are employed, and the formulations are conducted on arbitrary-shaped mesh cells. Then, the developed CFD method was verified by simulating a rising bubble in liquid under Bhaga et al.'s experimental conditions. Finally, the CFD method was validated by simulating the GE in the basic experiment, and the occurrence of the GE under the experimental GE condition was successfully reproduced.