To readily obtain a transparent culture medium containing no growth inhibitor and not requiring active carbon and expensive medicine and useful for differentiation, etc., of bacterium of the genus Legionella causing pneumonia by using gellan gum as a carrier for culture medium.

Gellan gum comprising polysaccharide composed of recurring unit of four sugar molecules of glucose-glucuronic acid-glucose-rhamnose represented by the formula obtained by separating and purifying polysaccharide which Pseudomonas elodea produces on the outside of the microbial cell is used as a carrier for culture medium and 0.5-1.5 W/V%, especially 0.8-1 W/V% gellan gum is formulated in the culture medium to readily provide the objective transparent culture medium for culturing bacteria of the genus Legionella, not requiring an active carbon or an expensive agent for absorbing a growth inhibitor, because the growth inhibitor contained in a conventional agar culture medium is not present and capable of differentiating the bacteria of the genus Legionella by adding a coloring substrate and fluorescent substrate which a bacterium of the genus Legionella produces, used as index of enzymatic activity to the medium.