A gate valve device having a case member (11, 12, 13) which can be attached to a pipe (k) and a gate valve body disposed in the case member (11, 12, 13) and capable of being inserted in the pipe (k) from a hole formed in a peripheral face of the pipe (k), wherein the gate valve body comprises a valve body core fitting, an elastic seal member covering the valve body core fitting, a press member which presses the elastic seal member from the insertion direction to be elastically deformed, when the elastic seal member is inserted in the pipe (k), a drive shaft formed in a rod shape along the insertion direction to move the press member along the insertion direction, and a movable member fitted in the middle in the longitudinal direction of the valve body core fitting and pressing, from the inside, the elastic seal member against an inner wall face of the pipe (k) in a cross direction crossing the insertion direction, the movable member is comprised of an arm having a second inclined section (4a1) receiving a pressing action from a first inclined section for converting a pressing direction, formed in the press member and pressing the elastic seal member against the inner wall face of the pipe (k), and a contact section which comes into contact with the inside of the elastic seal member to press the inside, characterised in that: a recess (4b2) is formed in the contact section of the movable member, and the elastic seal member is held within the interior portion of the recess (4b2), wherein the recess (4b2), formed in the contact section in the movable member, is formed in a groove shape extending in the vertical direction of the contact section, which vertical direction corresponds to said insertion direction, wherein the recess (4b2) has an opening angle (θ) at which a cross section of the recess (4b2) widens toward what would be the inner face of the pipe (k) when in use, wherein the opening angle (θ) of the recess (4b2) is between 50° to 100°, preferably between 60° and 90°, and the front end of the contact section is formed in a curved shape, such that when the elastic seal member is pressed against the inner wall of the pipe (k), the elastic seal member in the recess (4b2) can be smoothly pushed without damaging the elastic seal member. a recess (4b2) is formed in the contact section of the movable member, and the elastic seal member is held within the interior portion of the recess (4b2), wherein the recess (4b2), formed in the contact section in the movable member, is formed in a groove shape extending in the vertical direction of the contact section, which vertical direction corresponds to said insertion direction, wherein the recess (4b2) has an opening angle (θ) at which a cross section of the recess (4b2) widens toward what would be the inner face of the pipe (k) when in use, wherein the opening angle (θ) of the recess (4b2) is between 50° to 100°, preferably between 60° and 90°, and the front end of the contact section is formed in a curved shape, such that when the elastic seal member is pressed against the inner wall of the pipe (k), the elastic seal member in the recess (4b2) can be smoothly pushed without damaging the elastic seal member.