An insert includes: an insert body that has an opening and has an inner cavity therein; a light guide member that is inserted into the inner cavity of the insert body and guides light emitted from a light source; a light emitting portion that emits light guided by the light guide member; a light absorption member that generates photoacoustic waves by absorbing the light emitted from the light emitting portion; and a proximal end portion that has an inlet of liquid and a chamber communicating with the inlet and the inner cavity of the insert body. The inner cavity has a space for flow of the liquid, and liquid injected through the inlet is capable of flowing from the opening of the insert body through the chamber and the space for flow of the liquid.