NOVELTY - An odorization installation (1) of biogas comprises inert gas supply source (4), odorization agent tank (3), inert gas piping (7) which connects inert gas supply source and odorization agent tank and guides inert gas from inert gas supply source in odorization agent tank, metering pump (5) which sends trace amount of odorization agent out of odorization agent tank, odorization agent piping (6) which injects trace amount of odorization agent into biogas pipe interior in which biogas circulates, and pressure-reduction valve (8) which keeps pressure of inert gas constant.    USE - An odorization installation of biogas (claimed).    ADVANTAGE - Injection rate and odorization concentration of odorization agent are stabilized.    DETAILED DESCRIPTION - An odorization installation (1) of biogas comprises inert gas supply source (4), odorization agent tank (3), inert gas piping (7) which connects inert gas supply source and odorization agent tank and guides inert gas from inert gas supply source in odorization agent tank, metering pump (5) which sends trace amount of odorization agent out of odorization agent tank, odorization agent piping (6) which injects trace amount of odorization agent into biogas pipe interior in which biogas circulates, and pressure-reduction valve (8) which keeps pressure of inert gas provided in inert gas piping and supplied in odorization agent tank constant, is provided in odorization agent piping, and has back pressure valve (9) which keeps pressure of exit side of metering pump constant. The back pressure valve is positioned close to biogas piping (21). The setting pressure of back pressure valve is greater than or equal to 1.8 times of the maximum pressure of biogas that circulates biogas piping.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of an odorization installation of biogas.    Odorization installation (1)    Odorization agent tank (3)    Inert gas supply source (4)    Metering pump (5)    Odorization agent piping (6)    Inert gas piping (7)    Pressure-reduction valve (8)    Back pressure valve (9)    Biogas piping (21)