A treatment device for use with an endoscope or guide member and a needle which is used to perform treatment in a body by being operated outside the body, comprising: said needle; a cap assembly configured to be arranged at the distal end portion of the endoscope or guide member and having a link mechanism actuated by an operation outside the body; a flexible transmission member coupled to said link mechanism, the flexible transmission member having a distal end portion which is capable of being inserted into the body through or adjacent the endoscope or guide member; and a needle holder arm which is adapted to hold the needle and is coupled to and is actuated by the link mechanism and is able to move in a first direction to cause the needle to puncture a tissue and in a second direction opposite said first direction; wherein said cap assembly has first and second mounting locations; characterized in that said link mechanism comprises a gear assembly having a first gear element and a second gear element meshing with said first gear element; wherein said first gear element is pivotally coupled at said first mounting location and coupled to said transmission member, and said second gear element is pivotally coupled at said second mounting location and coupled to said needle holder arm, such that movement of said transmission member results in movement of said needle holder arm; wherein said first and second mounting locations (353, 347) are defined on two arms (338a, 338b) wherein, in a first position, said needle holder arm holds the needle so that the tip of the needle is located between said two arms and spaced from a distal end face of the endoscope or guide member and, in a second position, said needle holder arm extends and holds the needle so that a tissue piercing tip of the needle is exposed. said needle; a cap assembly configured to be arranged at the distal end portion of the endoscope or guide member and having a link mechanism actuated by an operation outside the body; a flexible transmission member coupled to said link mechanism, the flexible transmission member having a distal end portion which is capable of being inserted into the body through or adjacent the endoscope or guide member; and a needle holder arm which is adapted to hold the needle and is coupled to and is actuated by the link mechanism and is able to move in a first direction to cause the needle to puncture a tissue and in a second direction opposite said first direction; wherein said cap assembly has first and second mounting locations; characterized in that said link mechanism comprises a gear assembly having a first gear element and a second gear element meshing with said first gear element; wherein said first gear element is pivotally coupled at said first mounting location and coupled to said transmission member, and said second gear element is pivotally coupled at said second mounting location and coupled to said needle holder arm, such that movement of said transmission member results in movement of said needle holder arm; wherein said first and second mounting locations (353, 347) are defined on two arms (338a, 338b) wherein, in a first position, said needle holder arm holds the needle so that the tip of the needle is located between said two arms and spaced from a distal end face of the endoscope or guide member and, in a second position, said needle holder arm extends and holds the needle so that a tissue piercing tip of the needle is exposed.