We have observed an oscillation in the current density-voltage (J-V) characteristics caused by resonant tunneling phenomena by means of a newly developed Si-based double barrier structure (DBS). In the DBS fabrication, a thin single-crystal Si plate is used as a quantum well on SIMOX (separation by implanted oxygen) substrate. In order to obtain the Si quantum well, an anisotropic wet chemical etching followed by thermal oxidation is performed. Particularly, a novel advanced shaped etching mask is used to form an extremely thin Si plate. The width of the plate exceeds the resolution limitation of lithography. Accordingly, a quantum well with a width of 9 nm has been successfully obtained. The electrical characteristics of the sample that has around 40 nm wide quantum well and 1.6 nm thick SiO2 barriers are investigated. Oscillation peaks are observed in the J-V characteristics at 4.0 K. The observed peaks are in good agreement with the calculated resonant tunneling diode model.