When conditions for an electron gun mainly represented by extraction voltage V1 and accelerating voltage V0 are changed, a charged particle beam is once focused on a fixed position by means of a condenser lens and a virtual cathode position is calculated from a lens excitation of the condenser lens at that time and the mechanical positional relation of the electron gun to set an optical condition. This makes it possible to enhance display quality in a case where the data signal line is subjected to the block-reversal driving. One flow reserve index obtained under the two conditions can be used to compute coronary flow reserve. The method includes burning the interdigital electrode structure clean by heating up the soot sensor after the upper current threshold has been reached; monitoring the measurement current while the interdigital electrode structure is being burned clean; and stopping the burning clean when the value of the measurement current has reached a lower current threshold.