A diagnostic agent for infectious diseases which is capable of distinguishing among different kinds of bacterial species and which allows simple and non-invasive measurement and/or imaging in a short period of time is provided and a screening method for a therapeutic agent for infectious diseases caused by microorganisms are provided.A diagnostic agent for infectious diseases caused by nitroimidazole susceptible microorganisms, containing an imidazole derivative or a fused imidazole derivative having at least one nitro group on an imidazole ring, or a labeled form thereof as an active ingredient is provided.
