Abstract We present the manifestly covariant canonical operator formalism of a Weyl invariant (or equivalently, a locally scale invariant) gravity whose classical action consists of the well-known conformal gravity and Weyl invariant scalar-tensor gravity, on the basis of the Becchi-Rouet-Stora-Tyupin (BRST) formalism. It is shown that there exists a Poincaré-like IOSp(8|8) global symmetry as in Einstein’s general relativity, which should be contrasted to the case of only the Weyl invariant scalar-tensor gravity where we have a more extended Poincaré-like IOSp(10|10) global symmetry. This reduction of the global symmetry is attributed to the presence of the Stückelberg symmetry.