To provide an auxiliary air tank not to occupy a wide installation space.

A plurality of long cylindrical tanks 2 are arrayed in juxtaposition and a lateral beam 3 is welded on the upper and lower surfaces of a plurality of tanks 2 for intercoupling. The vent ports of a plurality of the tanks 2 are interconnected through a branch connection pipe 4, and a hose connection plug 5 is mounted on the vent port of the branch connection pipe 4. Collapsible legs 9 are mounted on a lateral beam on the under surface of the tank and a shading cover 10 is mounted on the upper surface of the tank 2. By installing an air compressor C between front and rear legs, the air compressor and this auxiliary air tank are installed on almost the same occupying area as the installation space of an air compressor alone. The cover 10 shields rain and direct sunlight and prevents the occurrence of a trouble of an air compressor due to overhead flooding and lowering of the efficiency of a compressor due to a solar heat.