To provide an intravital indwelling object capable of effectively preventing inflow of a fluid in a direction of preventing the flow by controlling gapping caused by displacement inside a tubular region of a living body where the fluid flows, and capable of preventing displacement less invasively; and an assembly including the intravital indwelling object.

An intravital indwelling object 100 expands inside a tubular region B of a living body where a fluid flows, and permits the flow of the fluid in one direction along the tubular region while preventing the flow in a direction opposite to the one direction. The intravital indwelling object includes: a holding part 102 which expands in such a way as to press the inner surface of the tubular region and which holds the intravital indwelling object to the tubular region; an adhesion part 105 provided on the outer surface of the holding part and capable of increasing the adhesive force to cells by receiving light; and a light guide part 106 which is provided on the outer surface of the holding part and at least part of which is exposed from the adhesion part to transmit light.