Deterioration of an InBO3:Tb phosphor screen deposited in potassium silicate solution was studied by a demountable apparatus designed to simulate the tube condition. Evolution of oxygen from the phosphor crystal was observed under intense electron irradiation. It was found that the deterioration is accelerated by phosphor screen heating, which depends on the screen weight, packing density, and an amount of silica gel deposited between particles and the surface of the screen.