An installation structure adapted to allow selection between the presence and the absence of a braking mechanism for braking an output shaft of a motor in a power tool in which a front housing is mounted frontwardly of a tubular motor housing accommodating the motor, characterized in that the installation structure comprises: a plurality of shaft support plates configured to support an output shaft of the motor and adapted to be provided between the front housing and the motor housing, the plurality of shaft support plates including a first shaft support plate adapted to be connected directly to the motor housing and a second shaft support plate adapted to be connected to the motor housing with a tubular spacer being provided between the second shaft support plate and the motor housing, a plurality of front shafts including a first front shaft adapted to be rotatably supported by the first shaft support plate and a second front shaft adapted to be rotatably supported by the second shaft support plate, the second front shaft being longer than the first front shaft in an axial dimension, the first and second front shaft being detachably connectable to a rear shaft of the output shaft, the rear shaft being disposed rearwardly of the front shaft, the braking mechanism including a flange plate, a braking member and a biasing member, and is configured to be mounted to the second shaft support plate and the second front shaft such that the flange plate is fixed to the second front shaft, the braking member is movable between a braking position in which the braking member is pressed against the flange plate and a retreating position in which the braking member is separate from the flange plate, and the biasing member is configured to press the braking member toward the flange plate, whereby two modes of configuration are selectable which include: a first mode without the braking mechanism in which the first front shaft is connected to the rear shaft with the first shaft support plate being mounted between the motor housing and the front housing; and a second mode with the braking mechanism in which the second front shaft is connected to the rear shaft with the second shaft support plate and the spacer being mounted between the motor housing and the front housing. a plurality of shaft support plates configured to support an output shaft of the motor and adapted to be provided between the front housing and the motor housing, the plurality of shaft support plates including a first shaft support plate adapted to be connected directly to the motor housing and a second shaft support plate adapted to be connected to the motor housing with a tubular spacer being provided between the second shaft support plate and the motor housing, a plurality of front shafts including a first front shaft adapted to be rotatably supported by the first shaft support plate and a second front shaft adapted to be rotatably supported by the second shaft support plate, the second front shaft being longer than the first front shaft in an axial dimension, the first and second front shaft being detachably connectable to a rear shaft of the output shaft, the rear shaft being disposed rearwardly of the front shaft, the braking mechanism including a flange plate, a braking member and a biasing member, and is configured to be mounted to the second shaft support plate and the second front shaft such that the flange plate is fixed to the second front shaft, the braking member is movable between a braking position in which the braking member is pressed against the flange plate and a retreating position in which the braking member is separate from the flange plate, and the biasing member is configured to press the braking member toward the flange plate, whereby two modes of configuration are selectable which include: a first mode without the braking mechanism in which the first front shaft is connected to the rear shaft with the first shaft support plate being mounted between the motor housing and the front housing; and a second mode with the braking mechanism in which the second front shaft is connected to the rear shaft with the second shaft support plate and the spacer being mounted between the motor housing and the front housing.