Time of flight method in the form of ion acceleration by a negative square pulse is investigated for lead ion plasma generated from a short burning time impulse vacuum arc. In the method, lead ions existing between the grid with earth potential and the accelerating grid which is connected to the ion accelerating tube are accelerated into the tube by applying a negative square pulse voltage to the accelerating grid. The ions fly toward the ion collector with a constant velocity dependent on the charge state. The effect of an axial magnetic field in the accelerating tube on the loss of the flying ions and on the obtained rate of the densities of Pb⁺¹ and Pb⁺² ions in the arc plasma is discussed. The relation between the amount of the accelerated ions and the accelerating voltage is also investigated from the viewpoint of time dependent propagation of the ion sheath, which is formed in front of the accelerating grid.