PURPOSE: To prevent breakdown of an outer casing even if a load is applied to a collector due to an external factor such as snow and the like, by coupling together the side walls, positioned facing and opposite to each other, of the outer casing containing the collector, by means of reinforcing devices.

CONSTITUTION: Reinforcing devices 4, which are made of metal plates bent to form, are placed to an box-form outer casing 2 which has an open surface and is thin in vertical direction. Bosses or rivets, protruding from the inner periphery of the outer casing 2, are inserted through holes formed in leg pieces 11 to secure each of the leg pieces 11 in a specified position. A leg 5a is installed to the outer surface of the outer casing 2 positioned facing and opposite to a position where the leg piece 11 is secured, the leg 5a and the reinforcing device 4 are coupled together by means of a bolt 15, and a side wall 3 of the outer casing 2 is positioned between the nip formed by the leg and the reinforcing device. Since, as noted above, the side walls of the outer case housing a collector are linked on the inner surface side of the outer casing by the reinforcing device, even if load produced due to an external factor such as snow, a wind pressure, earthquake, is applied to the collector, the outer casing is prevented from deformation or breakdown.