A power conversion system (200A) incorporated in a flight vehicle includes a power converter (10) having a semiconductor element stored therein. An energy storage (20) is disposed above the power converter (10) in the vertical direction, the energy storage storing a solid or liquid energy source containing a neutron attenuating material. The power converter (10) is provided with a movable mechanism (60) that moves to shift in position according to a tilt of the flight vehicle. The movable mechanism includes slider rails and a slider movable unit.