A manually operated hammer drill (1) includes a spindle (3) that drives a chuck (5) in a rotary manner about an axis of rotation (6) in a drilling mode, and includes a pneumatic hammer mechanism (4), that hammers against a tool inserted in the chuck (5) in a hammer mode. The hammer mechanism (4) has a piston (10) which performs reciprocating movements (12) parallel to the axis of rotation (6) in a cylinder (11) conformed inside the spindle (3) when hammer mode is activated. The piston (10) has at least one radial groove (21) in which an annular seal (22) is arranged.