  A canonical quantization for two dimensional gravity models, including a
dilaton gravity model, is performed in a way suitable for the light-cone gauge.
We extend the theory developed by Abdalla {\it et.al.}\cite{AM} and obtain the
correlation functions by using the screening charges of the symmetry algebra.
It turns out that the role of the cosmological constant term in the Liouville
theory is played by the screening charge of the symmetry algebra and the
resulting theory looks like a chiral part of the Liouville theory or a
non-critical open string theory. Moreover, we show that the dilaton gravity
theory has a symmetry realized by the centrally extended Poincar\'{e} algebra
instead of the $\slr$ Kac-Moody algebra which is a symmetry of an ordinary two
dimensional gravity theory.
