A strictly combined BEM-VOF model has been developed by making the best use of both method's merits. To carry out stable and accurate computations, the water surfaces obtained from BEM and VOF are smoothly connected in the transmission domain at each time step by using weighted means, and the velocity potential and its normal derivative in BEM and the value of the function of the volume of fluid in VOF are also corrected. The validity of the model was confirmed by comparing numerical results with theoretical values for Stokes' 5th order wave propagation through three domain BEM-VOF-BEM calculation as well as two domain BEM-VOF calculation. Wave propagation for a random waves through two domains was also checked. The model makes it practically possible to calculate wave-structure interactions under random waves by reducing the computational time and minimizing the numerical dissipation which is inevitable especially in single VOF calculations.