To provide a sighting device for a 89 type 5.56 mm rifle for preventing the occurrence of a difference between a sighting point and an impact point after a zero-in adjustment.

This sighting device 1 for the 89 type 5.56 mm rifle has a dot sight 10 and a mount part 20 for installing the dot sight 10 in the 89 type 5.56 mm rifle 2. The dot sight 10 has a columnar holding object part 12. The mount part 20 has a body part 21 fixed to a basic base 3 formed in the 89 type 5.56 mm rifle 2, and a cylindrical holding part detachably holding the holding object part 12 of the dot sight 10 and integrally formed with the body part 21. The dot sight 10 and the mount part 20 have respectively planar abutting parts 17a and 26a. The abutting part 17a of the dot sight 10 and the abutting part 26a of the mount part 20 abut when the holding object part 12 of the dot sight 10 is held by the holding part of the mount part 20.