The lava flow simulation code LavaSIM has been developed to give accurate predictions of volcanic disasters and to support evacuation plans. The code uses three-dimensional analysis with free surface evaluation, including boundary transport between the melt and the crust. It is therefore applicable to various types of lava flows and flow behaviors such as flood basalts, subaqueous lava flows, and levee formation. Heat transfer between the lava and the ground, air, and water, and between the melt and crust of the flow is calculated by using appropriate relations. The code has been verified by applying it to actual lava flows, and the simulation results have been compared with observations of the Izu-Osima flows of 1986. The lava flow rate, temperature, and properties were assigned values based on data from existing literature. The comparisons demonstrated the code's capability for prediction of inundated areas and maximum flow lengths. The present code is expected to be useful for real-time prediction during eruptions, in addition to assessing volcanic hazards and designing protection structures for existing installations against possible future lava flows.