We fabricated and characterized a silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistor with a SOI film thickness of 26 nm. The measured data at room temperature showed that the transconductance peak in the front gate bias shifted and then split into two peaks as the back gate voltage (VBS) decreased. The VBS regime of this splitting was opposite to that of thick SOI devices, suggesting a different physical mechanism in our thin device. Our device also showed VBS dependences of the threshold voltage and the subthreshold swing, which were different from those of thick SOI devices. Our data were explained by the volume inversion of a thin SOI in which both front and back gates strongly modulated the conduction band and charge state of the SOI. The splitting was interpreted as due to the population of the second subband of a triangular quantum well.