Fluid operated drilling device for drilling a hole, said drilling device having a hammer for creating the hole with a rotating and percussive motion, a rotation device for rotating the hammer and a drill rod connecting the rotation device to the hammer and transporting operating pressurized drill fluid to the hammer for creating the percussive motion of the hammer, the hammer comprising - a tubular main body having a hollow interior and an upper end and a lower end; - a back head, for connecting the hammer to the drill rod, coupled to the upper end of the main body and having a fluid pressure supply passage; - a cylindrical piston housing connected to the main body; - a reciprocating piston slidably installed in the piston housing for impacting a drill bit of a bit unit installed at the lower end of the main body, the drill bit being movable for a predetermined length longitudinally relatively to the main body, the piston having a first end and a second end, the first end being closer to the drill rod than the second end, a hollow portion through which operating fluid is discharged, a first communication hole connected to the hollow portion and an annular pressurizing portion protruding on piston's outer circumferential surface, - a space between the piston and the piston housing divided by the annular pressurizing portion extending in radial direction of the piston into first space portion for elevating the piston and second space portion for striking the piston; - a valve unit for controlling fluid discharge from the second space portion through the first communication hole to the hollow portion, and; - a fluid pressure supply unit for supplying high pressure fluid delivered to the fluid pressure supply passage of the back head alternatively to the first space portion and the second space portion; wherein the rotation device is rotating the bit unit using the drill rod and the main body, characterized in that the hammer further includes a second space in the hollow interior of the main body between the piston and the main body in radial direction of the piston and between the piston housing and the bit unit in the axial direction of the piston and the piston further includes first communication channels from the hollow portion of the piston into the second space for discharging the fluid between the piston and the main body. - a tubular main body having a hollow interior and an upper end and a lower end; - a back head, for connecting the hammer to the drill rod, coupled to the upper end of the main body and having a fluid pressure supply passage; - a cylindrical piston housing connected to the main body; - a reciprocating piston slidably installed in the piston housing for impacting a drill bit of a bit unit installed at the lower end of the main body, the drill bit being movable for a predetermined length longitudinally relatively to the main body, the piston having a first end and a second end, the first end being closer to the drill rod than the second end, a hollow portion through which operating fluid is discharged, a first communication hole connected to the hollow portion and an annular pressurizing portion protruding on piston's outer circumferential surface, - a space between the piston and the piston housing divided by the annular pressurizing portion extending in radial direction of the piston into first space portion for elevating the piston and second space portion for striking the piston; - a valve unit for controlling fluid discharge from the second space portion through the first communication hole to the hollow portion, and; - a fluid pressure supply unit for supplying high pressure fluid delivered to the fluid pressure supply passage of the back head alternatively to the first space portion and the second space portion;