Transparent thin films of tin suboxide with high electro-conductivity were prepared by DC reactive sputtering on a Sn-O-2/Ar system without any dopant. The curves of current versus voltage applied to the tin target, the I-V curves, were observed to be continuous in this system and there was no avalanche-like state jump between the oxide and metal modes. The highly electro-conductive tin suboxide films were obtained in the transition mode and the conductivity changes in a Gaussian-like manner depending on the applied voltage. This profile shows that the deposited film changes from a well-oxidized tin oxide to a partially oxidized tin when the applied voltage is increased.