We propose an approach to quantify the source of volcano-seismic signals assuming possible source geometries. Such an assumption reduces the number of free parameters in a waveform inversion, so we can quantify the source of these signals observed by a small number of seismic stations. We test this method by using the synthesized waveforms radiated by a fluid-filled crack and with seismic data observed at Kusatsu-Shirane Volcano. We find that waveform data from 2 to 3 three-component seismic stations surrounding the source enable us to estimate the source mechanism and location in the present approach.