To form nanoclusters with a specified atomic structure, an electrical trapping technique of charged particles was developed that allows us to confine, grow, and mass selectively eject cluster ions. Theoretical calculations and experiments using inert gas ions indicate that the developed ion trap can confine ions with a wide range of mass values using the external field of an ac quadrupole, and send out the ions mass selectively through the internal region of the quadrupole. To demonstrate performance, Si6H12+ clusters were selectively grown using SiH4 as a starting material. In the trap, confined SiHx+ ions were allowed to react with neutral SiH4 molecules or SiHy radicals, and SinHx+ clusters were grown, from which Si6H12+ clusters were automatically ejected by tuning trapping parameters.