Abstract: An object is to suppress resonance phenomenon. A pair of reinforcing members (18) are fixed on a gate drive substrate (7) with spacers (37) interposed therebetween and upright portions (40) of the pair of reinforcing members (18) are fastened with screws on a side wall of a cathode flange. A spacer (118) is fixed on the gate drive substrate (7) and a projection (118a) of the spacer (118) is inserted in an engaging member (119) fixed on the bottom of the cathode fin electrode (5) and thus fixed on the bottom of the cathode fin electrode (5). The pair of upright portions (40) as the first and second supporting points and the projection (118a) as the third supporting point stably support the gate drive substrate (7) on the cathode fin electrode (5) without freedom of rotation at the three positions arranged to surround an opening (49).