We developed a newly designed wavelength modulation (WM) system for highly sensitive absorption spectroscopy. In our system, the WM is realized by yawing an output mirror in a monochromator. In order to control an amplitude Δλ of the WM in a wide range, we employed a forced vibration of a permanent magnet driven by a magnetic field of a solenoid. Our system has an advantage of that the WM amplitude Δλ can be adjusted in extensively wide range from 0.08 nm to 11 nm only by tuning a driving frequency of the applying current to the solenoid, because we utilize a resonance phenomenon of the forced vibration for adjustment of the WM amplitude. By using our system, we measured WM absorption spectra of a Cu(2)O thin film and found clearly spectral structures for weak 2-4P excitonic resonances in the WM absorption spectra.