Japan Atomic Energy Agency and High Energy Accelerator Research Organization are constructing a MW-scale neutron source system, which will use liquid-mercury as a target material for spallation and to cool the target vessel in Japan Proton Accelerator Research Complex (J-PARC). The severe cavitation erosion may occur in the vessel of the target and it is necessarily to predict the life time of the target vessel, which is exposed to cavitation impact. As the target vessel is very thin and experimental erosion data induced by proton beam is very few, it is required to estimate the incubation period by a few points. The prediction method of incubation period has been proposed. In order to estimate cavitation erosion intensity on the target, an off-line test, which is modified on-beam test, was carried out. Measuring the cavitation intensity of the off-line test using a laser Doppler vibrometer, the relation between the cavitation intensity and developing time of cavitation bubble was found. The relation revealed a similarity law on scaling effect of cavitation erosion, which can be applied to cavitation erosion of pumps and turbines.