A steam generator tube rupture (SGTR) incident occurred in Mihama Unit 2 on February 9, 1991. This is the first SGTR incident where the emergency core cooling system was actuated in Japan. With some measures being taken to improve the model, the precise simulation of the SGTR using RETRAN-02/MOD4 was conducted. A nonequilibrium model was applied to the secondary side of the damaged SG to obtain a reasonable response of the secondary pressure. The model of the reactor upper head was improved to simulate void generation during reactor coolant system depressurization, and there was good agreement with the actual plant data.