To obtain a new biological indicator capable of inexpensively and readily producing and conveniently handling and providing a reliable and consistent result in the examination or monitoring of a specific sterilization step.

A biological indicator 10 is formed from two members (a sheet in which one part is folded on the other part) mutually hermetically sealed and forming a passage having three parts. One block is opened toward the outside and suitably formed as a meandering passage 30. A brittle ampule 14 is housed on the downstream of center passage part in the meandering passage. The brittle ampule 14 houses a culture medium for microorganisms. A carrier 12 in which microbial spores are inoculated is housed in an internal passage part (most downstream from the outside). The biological indicator 10 has preferably a passage formed by heat formation of thermoforming sheet and hermetical sealing is prepared by a heat-sealing method.