Thin film transistors (TFTs) with a small subthreshold slope (SS) are urgently required for low-voltage operating circuits on large-area and flexible substrates. Using InGaZnO deposited at room temperature (RT), we achieved 63 mV/dec, the smallest-ever SS reported for oxide semiconductor TFTs. To achieve the small SS as well as small Ioff, a fully-depleted off-state was employed by thinning the channel layer to 6 nm. Scalability down to L = 2 mum was confirmed. For Vg = 0 to 1.5 V operation, Ioff <10-17 A/mum and on/off ratio > 108 were obtained.