-1- BACKGROUND OF THE INVENTION This invention relates to a power converter apparatus using switching devices. Power converter apparatuses have gained a wide application in recent years to rotate A.C. motors at arbitrary rotating speeds. A power rectifier for normally converting an A.C. voltage to a D.C. voltage and a power inverter for inversely converting the D.C. voltage to the A.C. voltage are generally applied to the power converter apparatus and IGBT (Insulated Gate Bipolar Transistor) having a large capacity (high voltage and large current) and capable of switching at a high speed is employed in many case as a switching device (refer to JP-A-2010-178542, for example). SUMMARY OF THE INVENTION When the power converter apparatus is operated while an IGBT device of a main circuit is broken, however, the rupture of the broken IGBT device may invite scattering of internal members and generation of a conductive gas. When the conductive gas enwraps subsequently the IGBT devices and fuses, a short-circuit route having a potential exceeding a fuse rated voltage may develop. The power converter apparatuses of the prior art inclusive of the technology described in JP-A-2010-178542 cannot sufficiently cope with such a problem. In view of such a problem, the invention aims at providing a power converter apparatus which does not allow the short-circuit route to readily develop even when the broken IGBT undergoes rupture and the conductive gas is generated owning to the scatter of internal members. To accomplish this object, the power converter apparatus according to the invention employs the following construction. Namely, the invention provides a power converter apparatus that includes a plurality of switching devices connected in series between a positive side D.C. power source and a bus for outputting A.C. power; a plurality of switching devices connected in series between a negative side D.C. power source and a bus for outputting A.C. power; diode devices connected between a neutral point D.C. power source and the switching devices; and insulating walls for preventing the occurrence of a short-circuit route owing to a conductive gas, arranged in the proximity of the switching devices. -2- The invention can provide the power converter apparatus in which the short-circuit route does not readily occur even under the condition where the broken IGBT undergoes rupture and the conductive gas occurs owing to the scatter of the internal members. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a structural view showing a relation between a circuit of a power converter apparatus and an insulating wall in a first embodiment of the invention; FIG. 2 is a structural view showing a relation between a circuit of a power converter apparatus and an insulating wall in a second embodiment of the invention; FIGS. 3Ato 3C are perspective views each showing a plurality of structural surfaces when a main circuit of each phase is constituted into a cell unit in the power converter apparatus in the first embodiment of the invention; and FIGS. 4Ato 4C are perspective views each showing a plurality of structural surfaces when a main circuit of each phase is constituted into a cell unit in the power converter apparatus in the second embodiment of the invention. DESCRIPTION OF THE EMBODIMENTS The embodiments of the invention will be explained hereinafter with reference to the accompanying drawings. (First Embodiment) FIG 1 is a structural view showing the relation between the circuits of the power converter apparatus and the insulating walls in the first embodiment of the invention. The power converter apparatus of the first embodiment is used for a large capacity power converter apparatus for driving a main engine A.C. motor applied to a steel rolling plant, for example. To begin with, the circuit construction of the power converter apparatus will be explained, followed then by the explanation of the insulating walls in the power converter apparatus.