  A systematic scheme is developed for solving conformal algebra of the massive
vertex operators in the old covariant first quantized string theory. Using the
first massive level in the covariant spectrum of bosonic open string theory in
flat space-time as an example, we explicitly verify the conformal algebra for
the normal-ordered covariant vertex operator. We emphasize that the requirement
of fully covariant gauge-fixing specifies the form of normal-ordering for
massive vertex operators.
