On the development of short wavelength Free Electron Lasers, there is a issue of the extremelylarge system. It is caused by high energy accelerator and a long wiggler. In order to solve this, we proposea system utilizing a microwiggler and seeding X-rays. Microwiggler makes it possible to lase at the wavelength of vacuum ultraviolet to soft X-ray with low energy electron beam. With seeding X-ray, quick start-up and thus short wiggler length are expected. We examined the validity of seeding X-ray using computer simulation. As a result, when seeding X-ray of 10 kW average power is supplied, it was shown that wiggler length for the radiation saturation was shortened by 60 %. We are going to use Laser-produced Plasma X-ray (LPX) for the seeding X-ray source. The technique of using an ellipsoid mirror to improve coupling efficiency of LPX to seeding X-ray is also discussed.