  We propose a new gauge field theory which is an extension of ordinary string
field theory by assembling multiple state spaces of the bosonic string. The
theory includes higher spin fields in its massless spectrum together with the
infinite tower of massive fields. From the theory, we can easily extract the
minimal gauge invariant quadratic action for tensor fields with any symmetry.
As examples, we explicitly derive the gauge invariant actions for some simple
mixed symmetric tensor fields. We also construct covariantly gauge-fixed action
by extending the method developed for string field theory.
