Maintainability of a traversing in-core probe is improved without any hindrance to passage and the like in an access tunnel in which an indexing device of a TIP system is installed. An indexing device installation unit provided in the TIP system includes: a base on which an indexing device of a traversing in-core probe used in a nuclear reactor is mounted; a holding part holding the base in a suspended state in an access tunnel formed in a nuclear reactor containment vessel; and a lift mechanism moving up and down the base held by the holding part together with the indexing device.