To efficiently and easily perform the mounting work without increasing the load bearing even in a case when an interval of mounting legs is largely changed in accordance with the kinds of installations.

This mounting base opened downward and made out of a synthetic resin for loading and supporting the installation, is longitudinally divided into a first base 3 comprising a loading part 3a and a second base 4 comprising a loading part 4a, and both bases 3, 4 are connected by a connecting means A in a state that they can be expanded and contracted in the longitudinal direction.