An outer tube comprising: a penetration part configured to include a distal end, a proximal end and a longitudinal axis, and to penetrate a body wall; a first insertion hole (168, 268) in which a first insertion part of one of two medical instruments (10, 50) to be inserted inside the penetration part can be inserted in the inside of the penetration part along a direction parallel to the longitudinal axis so as to be freely movable back and forth; a second insertion hole (170, 270) which is disposed adjacent to the first insertion hole (168, 268) via a partition wall (172, 272), and in which a second insertion part of another one of the two medical instruments (10, 50) can be inserted in the inside of the penetration part along a direction parallel to the first insertion hole (168, 268) so as to be freely movable back and forth; a first slit type airtight valve which is provided in the first insertion hole (168, 268) and includes a first slit configured to maintain airtightness when the first insertion part is removed from the first insertion hole (168, 268); a second slit type airtight valve which is provided in the second insertion hole (170, 270) and includes a second slit configured to maintain airtightness when the second insertion part is removed from the second insertion hole (170, 270); a first opening type airtight valve which is provided in the first insertion hole (168, 268) and includes a first opening configured to closely contact with an outer peripheral part of the first insertion part to maintain airtightness when the first insertion part is inserted in the first insertion hole (168, 268); and a second opening type airtight valve which is provided in the second insertion hole (170, 270) and includes a second opening configured to closely contact with an outer peripheral part of the second insertion part to maintain airtightness when the second insertion part is inserted in the second insertion hole (170, 270), characterized in that, when the first insertion hole (168, 268) and the second insertion hole (170, 270) are projected on a surface orthogonal to the first insertion hole (168, 268), the first insertion hole (168, 268) and the second insertion hole (170, 270) are formed in a line-symmetric shape centering on a reference line passing through a center of the first insertion hole (168, 268) and a center of the second insertion hole (170, 270), and, in the surface orthogonal to the first insertion hole (168, 268), a distance from the center of the first insertion hole (168, 268) to a wall core of the partition wall (172, 272) is formed to be shorter than a distance from the center of the first insertion hole (168, 268) to an edge part thereof on a side opposite to the partition wall (172, 272), and a distance from the center of the second insertion hole (170, 270) to the wall core of the partition wall (172, 272) is formed to be shorter than a distance from the center of the second insertion hole (170, 270) to an edge part thereof on a side opposite to the partition wall (172, 272); and the first slit and the second slit are disposed along the reference line on the surface orthogonal to the first insertion hole (168, 268). a penetration part configured to include a distal end, a proximal end and a longitudinal axis, and to penetrate a body wall; a first insertion hole (168, 268) in which a first insertion part of one of two medical instruments (10, 50) to be inserted inside the penetration part can be inserted in the inside of the penetration part along a direction parallel to the longitudinal axis so as to be freely movable back and forth; a second insertion hole (170, 270) which is disposed adjacent to the first insertion hole (168, 268) via a partition wall (172, 272), and in which a second insertion part of another one of the two medical instruments (10, 50) can be inserted in the inside of the penetration part along a direction parallel to the first insertion hole (168, 268) so as to be freely movable back and forth; a first slit type airtight valve which is provided in the first insertion hole (168, 268) and includes a first slit configured to maintain airtightness when the first insertion part is removed from the first insertion hole (168, 268); a second slit type airtight valve which is provided in the second insertion hole (170, 270) and includes a second slit configured to maintain airtightness when the second insertion part is removed from the second insertion hole (170, 270); a first opening type airtight valve which is provided in the first insertion hole (168, 268) and includes a first opening configured to closely contact with an outer peripheral part of the first insertion part to maintain airtightness when the first insertion part is inserted in the first insertion hole (168, 268); and a second opening type airtight valve which is provided in the second insertion hole (170, 270) and includes a second opening configured to closely contact with an outer peripheral part of the second insertion part to maintain airtightness when the second insertion part is inserted in the second insertion hole (170, 270), characterized in that,