The aim of the present study is to clarify a scaling law for volcanic explosion earthquakes excited by a counter force of eruption. Based on the data of explosion earthquakes at three volcanoes, we derived the following results: The peak amplitude of the single force is proportional to the square of the pulse width of source time function. The initial pressure stored in a shallow region beneath a volcano before eruption is 1 MPa with a perturbation of one order. The kinetic energy of explosion earthquake is proportional to the cube of crater radius. The scaling law of the explosion earthquake is quite similar to that of the earthquake caused by a faulting. It is possible to predict the strength of explosion earthquake by measuring the crater radius beforehand.