Si single-electron transistors (SETs) were used for highly sensitive ion detection. A multiple-island channel structure was adapted in the SET for room-temperature operation. Clear Coulomb oscillation and diamonds were observed at room temperature. Using the Coulomb oscillation, clear pH responses of drain current (Id)-gate voltage (Vg) characteristics were obtained despite the existence of Id noise. Because Coulomb oscillations have a possibility to increase the slope of Id over Vg near the half-maximum current of the peaks, high resolving power of ion, and/or biomolecule concentration can be expected. A Si-structure will make it possible to integrate the sensors on a single chip.