blow an inert gas instead of air as a diluent gas or to mix an inert gas with air and also to change at least the ratio of air to the inert gas to flow as a diluent gas the ratio of the inert gas in the diluent gas may be increased or the diluent gas may be changed to the inert gas only when an inflammable gas satisfying the set condition is detected this configuration has a scope for implementation even if the operator enters the vehicle maintenance booth 20 for example by pouring a helium gas from a position higher than the operator it becomes possible to ensure safe respiration of the operator and also to isolate hydrogen which is the lightest and remains rear the ceiling from air by wrapping the hydrogen with helium which is the next lightest gas lastly another modification will be described with reference to fig4 fig4 is a diagram almost identical to fig1 and like reference numerals are assigned to like components with repeated descriptions omitted a major difference between fig4 and fig1 is that a vehicle maintenance booth 158 whose ceiling wall 160 is formed in a tilted state is introduced the ceiling wall 160 is set in such a way that the ceiling becomes higher towards a center point then a thin tube 162 is disposed upward from the top position of the ceiling wall 160 also a hydrogen sensor 164 is mounted inside the thin tube 162 an air discharge device 170 is mounted on the inside of the ceiling wall 160 the air discharge device 170 includes air discharge sections 172 174 176 and 178 and each of these air discharge sections has an air discharge port and a fan subsequently.