A synthetic yarn splicer that splices one yarn and the other yarn formed of synthetic fibers, comprising: a yarn splicing portion that includes a passage which forms a space through which the one yarn and the other yarn are insertable, a slit which communicates with the passage and allows the one yarn and the other yarn to be insertable into the passage from an insertion direction orthogonal to a penetration direction of the passage, and an injection hole which opens to the passage and injects a fluid; and a pair of clamping mechanisms (20,30) that is provided at a position interposing the passage of the yarn splicing portion in the penetration direction of the passage and clamps each of the one yarn and the other yarn inserted through the space, wherein the passage has a circular shape when viewed from the penetration direction of the passage, wherein the injection hole is disposed on a first line which is orthogonal to the penetration direction of the passage and the insertion direction and passes through a center of the passage, wherein the pair of clamping mechanisms (20,30) clamps the one yarn and the other yarn so that a line connecting a pair of clamping positions (CP1,CP2) of the one yarn and the other yarn is located in a predetermined area of the passage when viewed from the penetration direction of the passage, and wherein the predetermined area is an area which is located at the side opposite to the slit with respect to the first line and is located at the side opposite to the injection hole with respect to a second line following the insertion direction and passing through the center characterized in that the predetermined area is an area between a first circular arc which is distant from the center by 0.5 mm and a second circular arc which is distant from the inner peripheral surface of the passage by 0.3 mm. a yarn splicing portion that includes a passage which forms a space through which the one yarn and the other yarn are insertable, a slit which communicates with the passage and allows the one yarn and the other yarn to be insertable into the passage from an insertion direction orthogonal to a penetration direction of the passage, and an injection hole which opens to the passage and injects a fluid; and a pair of clamping mechanisms (20,30) that is provided at a position interposing the passage of the yarn splicing portion in the penetration direction of the passage and clamps each of the one yarn and the other yarn inserted through the space, wherein the passage has a circular shape when viewed from the penetration direction of the passage, wherein the injection hole is disposed on a first line which is orthogonal to the penetration direction of the passage and the insertion direction and passes through a center of the passage, wherein the pair of clamping mechanisms (20,30) clamps the one yarn and the other yarn so that a line connecting a pair of clamping positions (CP1,CP2) of the one yarn and the other yarn is located in a predetermined area of the passage when viewed from the penetration direction of the passage, and wherein the predetermined area is an area which is located at the side opposite to the slit with respect to the first line and is located at the side opposite to the injection hole with respect to a second line following the insertion direction and passing through the center characterized in that