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 (53e, 53e) 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 (53e, 53e) 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 (53e, 53e). 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 (53e, 53e) 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 (53e, 53e) 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 (53e, 53e).