To provide an in hole hammer which is short and compact so that the cost for a work head can be small.

An in hole hammer to bore the ground is provided with a slender hollow housing 10 to carry a built-in striking piston 24 in a piston chamber 22. A replaceable boring head is screw-connected to a forward end of the housing 10. The striking piston 24 gives an impact to a forward wall (forming the piston chamber 22) of the housing 10, and the impact energy for forward drive is transmitted to the boring head. The boring rod to push the in hole hammer forward and to turn the hammer around the axis in the longitudinal direction is connected to a rear end of the housing 10 through a connection piece 54. The connection piece 54 is fixed to the housing 10 in a slidable and non- rotational manner. The impact energy in the axial direction brought by the striking piston 24 is transmitted to the boring rod.