A medical instrument holder for holding a medical instrument comprising: an adapter connected to a base portion via an arm； and an instrument holding portion configured to be capable of holding the medical instrument and connected to the adapter. The instrument holding portion includes: a positioning portion configured to perform positioning with respect to the adapter when mounted on the adapter； and a rod configured to be connected to the positioning portion and to be inserted into the adapter. The adapter includes: a contact portion to which an upper surface of the positioning portion contacts and which has an opening； a fitting hole configured to communicate with the opening of the contact portion and to extend in an insertion direction of the rod； and a window portion that is open to the fitting hole and is configured so that an insertion state of the rod can be visually recognized through the window portion when the positioning portion contacts to the contact portion.
