Gas-solid flow is often encountered in chemical engineering. For better understanding the phenomena related to the gas-solid flows, numerical simulation is used frequently. The DEM-CFD method, where discrete element method (DEM) and computational fluid dynamics (CFD) are respectively employed in solid and gas phases, is a standard method for the gas-solid flow simulations. In previous studies, adequacy of the DEM-CFD method was proven through verification and validation. Although the DEM-CFD method was established, stable calculations could not be substantially performed in the systems, where drag force acting on a solid particle became extremely large. Unfortunately we can easily encounter these systems in actually industrial systems such as spouted bed, circulating fluidized bed, etc. On the other side, we cannot predict the drag force in advance, and hence extremely small time step might be used when we calculate these systems. At present, efficient calculation algorithm is required to calculate the gas-solid flow systems, where the drag force become huge. From this background, we newly develop an implicit algorithm for drag force term in the DEM-CFD method. Validation tests were performed in several gas-solid systems. The simulation results well agreed with the experimental ones. Incidentally, in these systems, the calculation could not be implemented without the implicit algorithm. Consequently, adequacy of the implicit algorithm was shown thorough this study.