A surgical instrument holding device comprising: an adapter from which a surgical instrument is detachable; a separator from which the adapter is detachable; a holding body from which the separator is detachable; and a driving body configured to support the holding body in a rotatable manner and apply a driving force to the holding body, wherein the separator is provided with a receiving portion, the adapter is provided with an engaging portion that is elastically deformed at a time of attachment of the adapter to the separator and a pressing portion that is pressed against the receiving portion at the time of attachment of the adapter to the separator, and the adapter is attached to the separator by pressing of the pressing portion against the receiving portion and engagement of the engaging portion with the receiving portion from a direction opposite to a direction in which the pressing portion presses the receiving portion in a state in which the engaging portion is elastically restored, wherein: the engaging portion is formed in a shape of projecting from the pressing portion; the engaging portion is provided with a proximal end portion that is continuous with the pressing portion and that extends in a direction of projecting from the pressing portion, a folded portion continuous with the proximal end portion and formed in a folded shape, an outside portion continuous with the folded portion and positioned on a peripheral side of the pressing portion, at least a part of the outside portion being opposed to the proximal end portion, and an engaging pawl portion continuous with the outside portion and bent with respect to the outside portion in a direction of approaching the pressing portion; and the engaging pawl portion is engaged with the receiving portion. an adapter from which a surgical instrument is detachable; a separator from which the adapter is detachable; a holding body from which the separator is detachable; and a driving body configured to support the holding body in a rotatable manner and apply a driving force to the holding body, wherein the separator is provided with a receiving portion, the adapter is provided with an engaging portion that is elastically deformed at a time of attachment of the adapter to the separator and a pressing portion that is pressed against the receiving portion at the time of attachment of the adapter to the separator, and the adapter is attached to the separator by pressing of the pressing portion against the receiving portion and engagement of the engaging portion with the receiving portion from a direction opposite to a direction in which the pressing portion presses the receiving portion in a state in which the engaging portion is elastically restored, wherein: the engaging portion is formed in a shape of projecting from the pressing portion; the engaging portion is provided with a proximal end portion that is continuous with the pressing portion and that extends in a direction of projecting from the pressing portion, a folded portion continuous with the proximal end portion and formed in a folded shape, an outside portion continuous with the folded portion and positioned on a peripheral side of the pressing portion, at least a part of the outside portion being opposed to the proximal end portion, and an engaging pawl portion continuous with the outside portion and bent with respect to the outside portion in a direction of approaching the pressing portion; and the engaging pawl portion is engaged with the receiving portion. the engaging portion is formed in a shape of projecting from the pressing portion; the engaging portion is provided with a proximal end portion that is continuous with the pressing portion and that extends in a direction of projecting from the pressing portion, a folded portion continuous with the proximal end portion and formed in a folded shape, an outside portion continuous with the folded portion and positioned on a peripheral side of the pressing portion, at least a part of the outside portion being opposed to the proximal end portion, and an engaging pawl portion continuous with the outside portion and bent with respect to the outside portion in a direction of approaching the pressing portion; and the engaging pawl portion is engaged with the receiving portion.