An administration apparatus that administers a substance to be administered to an administration target area, and the administration apparatus comprising: an apparatus body having a through hole that is formed in an axial direction; a piston portion disposed so as to be slidable in the through hole; a syringe portion disposed on a side of a tip of the apparatus body, the syringe portion having an accommodation chamber that is capable of accommodating the substance to be administered, a plunger that pressurizes the substance to be administered in the accommodation chamber in response to a slide of the piston portion, and a nozzle portion that includes a channel in which the substance to be administered in the accommodation chamber pressurized by the plunger flows, and ejects the substance to be administered from an ejection outlet formed at a tip of the channel; an ignition device that is disposed on a side of a base end of the apparatus body, that combusts gunpowder, and that supplies ejection energy for ejecting the substance to be administered from the nozzle portion using gunpowder combustion in the ignition device; and a sealing member that is formed of a metal plate-like member, that separates a space in the apparatus body into a first space in which the ignition device is disposed and a second space in which the piston portion is disposed, and that seals a combustion product generated by the ignition device in the first space, wherein a peripheral edge portion of the sealing member is fixed to an inner wall that defines the space in the apparatus body, and the sealing member is deformed by the gunpowder combustion in the ignition device such that a central portion of the sealing member is displaced to a side of the piston portion, to thereby press the piston portion against the plunger using the central portion. an apparatus body having a through hole that is formed in an axial direction; a piston portion disposed so as to be slidable in the through hole; a syringe portion disposed on a side of a tip of the apparatus body, the syringe portion having an accommodation chamber that is capable of accommodating the substance to be administered, a plunger that pressurizes the substance to be administered in the accommodation chamber in response to a slide of the piston portion, and a nozzle portion that includes a channel in which the substance to be administered in the accommodation chamber pressurized by the plunger flows, and ejects the substance to be administered from an ejection outlet formed at a tip of the channel; an ignition device that is disposed on a side of a base end of the apparatus body, that combusts gunpowder, and that supplies ejection energy for ejecting the substance to be administered from the nozzle portion using gunpowder combustion in the ignition device; and a sealing member that is formed of a metal plate-like member, that separates a space in the apparatus body into a first space in which the ignition device is disposed and a second space in which the piston portion is disposed, and that seals a combustion product generated by the ignition device in the first space, wherein a peripheral edge portion of the sealing member is fixed to an inner wall that defines the space in the apparatus body, and the sealing member is deformed by the gunpowder combustion in the ignition device such that a central portion of the sealing member is displaced to a side of the piston portion, to thereby press the piston portion against the plunger using the central portion.