  A two dimensional hydrochemical hybrid code, KM2, is constructed to deal with
astrophysical problems that would require coupled hydrodynamical and chemical
evolution. The code assumes axisymmetry in cylindrical coordinate system, and
consists of two modules: a hydrodynamics module and a chemistry module. The
hydrodynamics module solves hydrodynamics using a Godunov-type finite volume
scheme and treats included chemical species as passively advected scalars. The
chemistry module implicitly solves non-equilibrium chemistry and change of the
energy due to thermal processes with transfer of external ultraviolet
radiation. Self-shielding effects on photodissociation of CO and H$_2$ are
included. In this introductory paper, the adopted numerical method is
presented, along with code verifications using the hydrodynamics modules, and a
benchmark on the chemistry module with reactions specific to a photon-dominated
region (PDR). Finally, as an example of the expected capability, the
hydrochemical evolution of a PDR is presented based on the PDR benchmark.
