On Novemer 16, 1987 Izu-Oshima volcano erupted again after a 1 year quiescence. Beginning 20 seconds prior to the eruption, low-frequency waves with large amplitudes (1.6 cm/s, at most) were observed at Gojinka-chaya (GJK) which is located 1.8 km north-west of the central crater of the Izu-Oshima volcano. This station consists of a low gain velocity-type seismometer with wide frequency range and data have been recorded by a digital event recorder. The pre-emption seismogram was modeled as a set of point sources embedded below the central crater in order to obtain the mechanism of eruption or the generation process of low frequency waves. An inversion was performed to obtain 3 independent components, moment tensor (M_XX+M_YY), M_ZZ and single force F_Z. The results are as follows: there was an upward single force at shallow depth (0.5 km), a downward single force at middle depth (1.0-1.5 km), and an upward single force at the deepest (2.0 km). This set of forces is interpreted to represent the draining of magma (or hot dry rock) down through the vent. There was also a negative M_ZZ moment tensor component at shallow depth. This can be interpreted as a shrinking horizontal tensile crack caused by the initiation of magma migration. A remarkably large component of (M_XX+M_YY) was found at middle depth. This can be interpreted to mean that the vent was at a high pressure before the eruption.