PROBLEM TO BE SOLVED: To provide an operation instrument for osteosynthesises which can make smooth fitting/removing operation of a guide instrument to a length object.SOLUTION: When fitting/removing a pin guide instrument 23 which connects one pin guide 31 and the other pin guide 32 freely openably and closably by a connection part 33 to/from a guide pin 21, at an external of an outer cylinder instrument 22, half cut surfaces 34 and 34 of the pin guides 31 and 32 of the one and the other are opened to an interval exceeding a diameter of the guide pin 21, and accessing from a radial direction of the guide pin 21, each of fitting grooves 35 of the one and the other is fitted to the guide pin 21. When the guide pin 21 is guided with the pin guide instrument 23, the half cut surfaces 34 and 34 of the pin guides 31 and 32 of the one and the other are closed, inserted in the outer cylinder instrument 22 to be unified, thereby forming a guide hole of the guide pin 21 by the one fitting groove 35 and the other fitting groove 35.
