A vehicle pop-up hood device actuator, comprising: a main body section that is formed in a tube shape, and that internally includes a gas generator; a cylinder that is formed in a bottomed tube shape closed off at one axial direction side, that has an inner peripheral face at the one axial direction side configuring a general face, that covers a portion at the one axial direction side of the main body section, that is moved toward the one axial direction side of the main body section by gas generated by the gas generator to lift a hood of a vehicle to a lifted position, and that is configured so as to be capable of moving to a retention position further toward another axial direction side of the main body section than the lifted position; a sealing member that is formed in a ring shape, that is provided at an outer peripheral portion at one axial direction end portion of the main body section, and that forms a seal between the main body section and the general face; a retention member that is provided at the outer peripheral portion of the main body section at a position further toward the another axial direction side of the main body section than the sealing member, that abuts on the inner peripheral face of the cylinder, and that retains the cylinder by engaging with the cylinder at the retention position; and an enlarged diameter face that configures an inner peripheral face at the another axial direction side of the cylinder, that has an internal diameter set larger than both an internal diameter of the general face and an external diameter of the sealing member, and that is disposed at a radial direction outer side of the main body section with respect to the sealing member at the lifted position, characterized in that the enlarged diameter face is disposed at a radial direction outer side of the main body section with respect to the sealing member at the lifted position, so that gas inside the cylinder accordingly flows out from between the enlarged diameter face and the sealing member toward the opening side of the cylinder. a main body section that is formed in a tube shape, and that internally includes a gas generator; a cylinder that is formed in a bottomed tube shape closed off at one axial direction side, that has an inner peripheral face at the one axial direction side configuring a general face, that covers a portion at the one axial direction side of the main body section, that is moved toward the one axial direction side of the main body section by gas generated by the gas generator to lift a hood of a vehicle to a lifted position, and that is configured so as to be capable of moving to a retention position further toward another axial direction side of the main body section than the lifted position; a sealing member that is formed in a ring shape, that is provided at an outer peripheral portion at one axial direction end portion of the main body section, and that forms a seal between the main body section and the general face; a retention member that is provided at the outer peripheral portion of the main body section at a position further toward the another axial direction side of the main body section than the sealing member, that abuts on the inner peripheral face of the cylinder, and that retains the cylinder by engaging with the cylinder at the retention position; and an enlarged diameter face that configures an inner peripheral face at the another axial direction side of the cylinder, that has an internal diameter set larger than both an internal diameter of the general face and an external diameter of the sealing member, and that is disposed at a radial direction outer side of the main body section with respect to the sealing member at the lifted position, characterized in that the enlarged diameter face is disposed at a radial direction outer side of the main body section with respect to the sealing member at the lifted position, so that gas inside the cylinder accordingly flows out from between the enlarged diameter face and the sealing member toward the opening side of the cylinder.