A microwave-driven in-tube accelerator (MITA) concept was proposed in this paper to improve thrust performance of a microwave rocket driven by an intense electromagnetic wave beam. A beam focusing process, plasma propagation process, and thrust performance process were numerically reproduced by modeling the electromagnetic wave propagation, plasma reaction and transport, and shock wave propagation. A beam diffraction effect was removed and a clear beam focusing at the rear side of the vehicle was achieved when the beam having a higher frequency was selected. An energy-absorption rate by the plasma increased with an increase in the incident beam frequency because a cut-off density for wave reflection increased. The thruster obtained a positive thrust because of an interaction between the thruster and shock wave induced by the beam focusing. A momentum coupling coefficient in the MITA was evaluated as 30 N/MW by the shock wave simulation, which is comparable level as the laser-driven in-tube accelerator.