A suturing device for being inserted into and used in a body, the device comprising: a front arm having a string support portion supporting both ends of a surgical string , and a front guide portion attached to the string support portion ; and a back arm having a needle-shaped member , a needle support portion supporting the needle-shaped member in a state of pointing to a predetermined first direction, and a back guide portion attached to the front guide portion such that the back guide portion is capable of sliding, relative to the front guide portion , in a direction substantially parallel to the first direction, and rotating, relative to the front guide portion , around an axis substantially parallel to the first direction, wherein the front guide portion has a substantially tubular member, wherein the back guide portion is disposed inside the front guide portion , wherein the front guide portion has a first guide groove guiding the needle-shaped member to one end of the surgical string supported by the string support portion , and a second guide groove guiding the needle-shaped member to the other end of the surgical string , when the back arm slides in the first direction characterised in that the first guide groove and the second guide groove have guide through-grooves which extend in the direction substantially parallel to the first direction, and which are formed in a wall portion of the front guide portion such that the guide through-grooves extend from an inside to an outside of the substantially tubular member , wherein the needle support portion is slidably and freely fitted into the guide through-grooves . a front arm having a string support portion supporting both ends of a surgical string , and a front guide portion attached to the string support portion ; and a back arm having a needle-shaped member , a needle support portion supporting the needle-shaped member in a state of pointing to a predetermined first direction, and a back guide portion attached to the front guide portion such that the back guide portion is capable of sliding, relative to the front guide portion , in a direction substantially parallel to the first direction, and rotating, relative to the front guide portion , around an axis substantially parallel to the first direction, wherein the front guide portion has a substantially tubular member, wherein the back guide portion is disposed inside the front guide portion , wherein the front guide portion has a first guide groove guiding the needle-shaped member to one end of the surgical string supported by the string support portion , and a second guide groove guiding the needle-shaped member to the other end of the surgical string , when the back arm slides in the first direction characterised in that the first guide groove and the second guide groove have guide through-grooves which extend in the direction substantially parallel to the first direction, and which are formed in a wall portion of the front guide portion such that the guide through-grooves extend from an inside to an outside of the substantially tubular member , wherein the needle support portion is slidably and freely fitted into the guide through-grooves . <EOS>