The creation of a synthetic Rashba field in a Si channel of a Si spin metal-oxide-semiconductor field-effect transistor is introduced and discussed. The Rashba field is attributed to the local electric field at the Si/SiO₂ interface and can be tuned by a gate electric field. An effective magnetic field due to the Rashba field gives rise to spin-lifetime anisotropy in a Si channel, which is detected by the oblique Hanle effect. It is also found that the Rashba field is built-in even at a gate voltage of 0 V. The magnitude of the spin splitting energy is estimated to be 0.6 µeV, which is comparable to that in strained GaAs.