Extended string field theory for massless higher-spin fields

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.