To provide a convenient outdoor installation space having an interchangeability with various types of storage chamber bodies with depth dimensions varying from large to small ones and capable of applying to general purposes.

A plurality of synthetic resin nut holding tubes 40a, 40b, 40c, and 40d comprising flat-shaped concrete blocks with specified thickness dimension, lateral dimension, and depth dimension and having fixing nuts 42 sealingly formed integrally with the upper end opening edge parts thereof are buried vertically in the concrete block in the state of being dotted-distributed on right and left sides along the depth direction of the concrete block. Accordingly, the bottom plates 14 of the various types of storage chamber bodies A having depth dimensions varying from large to small ones can be interchangeably and fixedly assembled with the concrete block directly or through a separate support leg C by using assembly bolts 41 threadably tightened selectively to the fixing nuts 42 for the nut holding tubes 40a to 40d from the upper side.