A hammer drill, comprising a motor, an internal mechanism (113, 117) driven by the motor, a housing that houses the motor and the internal mechanism (113, 117), a hammer bit detachably coupled to one end of the housing in the longitudinal direction of the hammer bit via a tool holder and driven by the internal mechanism (113, 117) in the longitudinal direction of the hammer bit to perform a predetermined operation, the internal mechanism comprising a motion converting mechanism that converts the rotating output of the motor to linear motion and drives the hammer bit in its longitudinal direction and a power transmitting mechanism that appropriately reduces the speed of the rotating output of the motor and transmits the rotating output as rotation to the hammer bit, a handgrip connected to the other end of the housing in the longitudinal direction of the hammer bit, and a dynamic vibration reducer including a weight and an elastic element, the elastic element being disposed between the weight and the housing and adapted to apply a biasing force to the weight, wherein the weight reciprocates in the longitudinal direction of the hammer bit against the biasing force of the elastic element, whereby the dynamic vibration reducer reduces vibration which is caused in the housing in the longitudinal direction of the hammer bit in the working operation, wherein the dynamic vibration reducer is disposed by utilizing an internal space defined by the housing, wherein the one end of the housing is the front side and the other end of the housing is the rear side of the housing, the housing of the hammer drill includes a motor housing and a crank housing forming an outer housing, wherein the motor housing is located on a lower part of the handgrip toward the front side of the housing and houses the driving motor and the crank housing is located on an upper part of the handgrip toward the front side of the housing and joined to the motor housing from above such that the side of the driving motor is the lower side of the housing and the opposite side is the upper side of the housing in a vertical direction and a horizontal direction extends from a left side to a right side of the housing and the vertical direction and the horizontal direction are perpendicular to the longitudinal direction, wherein the weight is disposed within a vibration reducer body and can move in the longitudinal direction of the vibration reducer body, the elastic element is formed by biasing springs applying a spring force to the weight when the weight moves in the longitudinal direction, the weight comprises projections on the right and left sides of the weight in the middle in the longitudinal direction, the biasing springs are disposed between the projections and the front end and the rear end of the vibration reducer body, wherein the dynamic vibration reducer is disposed by utilizing the internal space existing between the inner wall surface of the upper region of the crank housing and the internal mechanism and the internal space is dimensioned to be larger in the horizontal and longitudinal directions than in the vertical direction and the dynamic vibration reducer has a shape conforming to the internal space.