A pressure reduction valve device comprising: a body that has a valve accommodation hole; a valve mechanism that is accommodated in the valve accommodation hole; a cylinder that is provided on the body downstream of the valve mechanism; a piston that is accommodated in the cylinder, the piston and the body defining a pressure reduction chamber leading to a gas outlet provided through the body; and a support member that is provided inside the body, that supports the valve mechanism from an upstream side, and that has a communication passage that is provided in such a manner as to establish communication between a gas inlet provided through the body and the valve accommodation hole, wherein the valve mechanism has a valve seat having a through-hole that serves as a gas flow channel, a valve body that is provided upstream of the valve seat and that has an urging member accommodation hole opening to an end portion of the valve mechanism on an opposite side of the valve seat, and an urging member that is interposed between a bottom surface of the urging member accommodation hole and the support member and that urges the valve body toward the valve seat, the support member is inserted in the urging member accommodation hole, has a distal end portion that supports the valve body via the urging member, and an inner introduction passage is provided between an outer peripheral surface of the distal end portion and an inner peripheral surface of an end portion of the valve body where the urging member accommodation hole opens, such that a gas dispersed to the inner introduction passage acts as a force that pushes up the valve body toward a downstream of the pressure reduction valve device, wherein the support member has a communication passage which penetrates a part of the support member in an axial direction of the support member, an outer introduction passage that leads to the pressure reduction chamber from the valve accommodation hole is provided between an outer surface of the valve body and an inner surface of the valve accommodation hole, the inner introduction passage and the outer introduction passage are opposed, in an axial direction thereof, to the communication passage, wherein the end portion of the valve body on a side where the urging member accommodation hole opens is provided such that a wall thickness thereof decreases as a distance to the upstream side decreases, a first taper is provided on an inner peripheral edge of an opening portion of the urging member accommodation hole, and a second taper is provided on an outer peripheral edge of an opening portion of the valve body, characterized in that the gradient of the first taper with respect to the axial direction is larger than the gradient of the second taper with respect to the axial direction. a body that has a valve accommodation hole; a valve mechanism that is accommodated in the valve accommodation hole; a cylinder that is provided on the body downstream of the valve mechanism; a piston that is accommodated in the cylinder, the piston and the body defining a pressure reduction chamber leading to a gas outlet provided through the body; and a support member that is provided inside the body, that supports the valve mechanism from an upstream side, and that has a communication passage that is provided in such a manner as to establish communication between a gas inlet provided through the body and the valve accommodation hole, wherein the valve mechanism has a valve seat having a through-hole that serves as a gas flow channel, a valve body that is provided upstream of the valve seat and that has an urging member accommodation hole opening to an end portion of the valve mechanism on an opposite side of the valve seat, and an urging member that is interposed between a bottom surface of the urging member accommodation hole and the support member and that urges the valve body toward the valve seat, the support member is inserted in the urging member accommodation hole, has a distal end portion that supports the valve body via the urging member, and an inner introduction passage is provided between an outer peripheral surface of the distal end portion and an inner peripheral surface of an end portion of the valve body where the urging member accommodation hole opens, such that a gas dispersed to the inner introduction passage acts as a force that pushes up the valve body toward a downstream of the pressure reduction valve device, wherein the support member has a communication passage which penetrates a part of the support member in an axial direction of the support member, an outer introduction passage that leads to the pressure reduction chamber from the valve accommodation hole is provided between an outer surface of the valve body and an inner surface of the valve accommodation hole, the inner introduction passage and the outer introduction passage are opposed, in an axial direction thereof, to the communication passage, wherein the end portion of the valve body on a side where the urging member accommodation hole opens is provided such that a wall thickness thereof decreases as a distance to the upstream side decreases, a first taper is provided on an inner peripheral edge of an opening portion of the urging member accommodation hole, and a second taper is provided on an outer peripheral edge of an opening portion of the valve body, characterized in that the gradient of the first taper with respect to the axial direction is larger than the gradient of the second taper with respect to the axial direction.