A medical tool holding articulated arm comprising: a plurality of articulations each including: a ball having a spherical surface; a housing accommodating the ball and having a front opening whose diameter is smaller than the diameter of the ball and a stopper bottom opposing the front opening; and a presser arranged between the stopper bottom and the ball and moved toward and away from the ball by a pressing mechanism (14, 15, P), wherein the plurality of articulations are connected in series with corresponding ends of the housing and ball of the adjacent articulations; and a tool holder for holding a tool attached to the ball of a foremost one of the plurality of articulations; wherein the pressing mechanism (14, 15, P) includes: a spring arranged between the stopper bottom and the presser to bias the presser toward the ball; an acting member coupled with a part of the presser behind the stopper bottom; and a sealed chamber defined between the acting member and the stopper bottom, wherein the acting member is moving together with the presser away from the ball when a gas is supplied into the sealed chamber; the tool holding articulated arm being characterized by a double pipe being flexible and passing through the plurality of articulations, the double pipe including an inner passage (a) and an outer passage (b) so that a compressed gas is supplied in one direction into the inner passage, is turned at an end of the inner passage into the outer passage, and is guided in the opposite direction through the outer passage; a valve arranged at the end of the inner passage and controlled to open and close the flow of the compressed gas; and a branch passage branched from the outer passage in each of the plurality of articulations and connected to the sealed chamber. a plurality of articulations each including: a ball having a spherical surface; a housing accommodating the ball and having a front opening whose diameter is smaller than the diameter of the ball and a stopper bottom opposing the front opening; and a presser arranged between the stopper bottom and the ball and moved toward and away from the ball by a pressing mechanism (14, 15, P), wherein the plurality of articulations are connected in series with corresponding ends of the housing and ball of the adjacent articulations; and a ball having a spherical surface; a housing accommodating the ball and having a front opening whose diameter is smaller than the diameter of the ball and a stopper bottom opposing the front opening; and a presser arranged between the stopper bottom and the ball and moved toward and away from the ball by a pressing mechanism (14, 15, P), wherein the plurality of articulations are connected in series with corresponding ends of the housing and ball of the adjacent articulations; and a tool holder for holding a tool attached to the ball of a foremost one of the plurality of articulations; wherein the pressing mechanism (14, 15, P) includes: a spring arranged between the stopper bottom and the presser to bias the presser toward the ball; an acting member coupled with a part of the presser behind the stopper bottom; and a sealed chamber defined between the acting member and the stopper bottom, a spring arranged between the stopper bottom and the presser to bias the presser toward the ball; an acting member coupled with a part of the presser behind the stopper bottom; and a sealed chamber defined between the acting member and the stopper bottom, wherein the acting member is moving together with the presser away from the ball when a gas is supplied into the sealed chamber; the tool holding articulated arm being characterized by a double pipe being flexible and passing through the plurality of articulations, the double pipe including an inner passage (a) and an outer passage (b) so that a compressed gas is supplied in one direction into the inner passage, is turned at an end of the inner passage into the outer passage, and is guided in the opposite direction through the outer passage; a valve arranged at the end of the inner passage and controlled to open and close the flow of the compressed gas; and a branch passage branched from the outer passage in each of the plurality of articulations and connected to the sealed chamber.