We have investigated the SiO2/4H-SiC interface in the metal-oxide-semiconductor (MOS) structure using positron annihilation spectroscopy. The oxide layer was fabricated by the pyrogenic method. The Doppler broadening of annihilation quanta were measured as functions of the incident positron energy and the gate bias. With negative gate bias, significant increase in S-parameter was observed. This indicates that the fraction of positrons reaching the aluminium gate electrode increases with the bias voltage. By ultraviolet (UV) light irradiation, S-parameter was found to decrease. Considering the fact that the width of depletion layer shrinks upon UV irradiation, the drift of positrons towards the aluminium electrode may be suppressed. Even after stopping UV irradiation, the suppressed S-parameter was found to be persistent in the darkness. When the MOS sample was released from the circuit and was kept more than one day, S-parameter was found to recover. The above effect implies that the electron-hole recombination rate after UV irradiation is rather moderated even in the darkness due to low interfacial states acting as recombination centers.