There are several methods of tracking gas-liquid interface of multi-phase flows (MF). In these methods, the MARS (Multi-interfaces Advection and Reconstruction Solver) developed by one of the authors has been successfully applied to many problems, because of the accurate volume/mass conservation of fluid. But the MARS was developed on a structured gird system, and it is difficult to get the results with high accuracy for the complex geometry of the solution domain. On the other hand, the usage of an unstructured grid system in the computational fluid dynamics is now to be a popular manner for handling the complex geometry of the computational domain. In the present stydy, we have developed an interface-volume tracking algorithm on the unstructured grid system for the MF numerical simulation based on the MARS. This paper describes the interface-volume tracking procedure based on the MARS for the unstructured grid system, and discusses on the accuracy of the reconstruction of the interfaces and also discusses on the conservation of the volume of fluid through the advection process.