An apparatus (14; 24; 36; 82) for estimating a quantity of state relating to a motor (8; 32), comprising: a motor model (18; 46; 84) that is adapted to reflect a characteristic of a motor (8; 32) and includes at least inputs pertaining to a first quantity of state and a second quantity of state relating to the motor (8; 32) and an output pertaining to a third quantity of state relating to the motor (8; 32); a comparator (20; 48) that is adapted to calculate a difference between the third quantity of state in an actual motor (8; 32) and the third quantity of state output from the motor model (18; 46; 84); and an amplifier (22; 52) that is adapted to amplify an output of the comparator (20; 48) at a predetermined gain, wherein the first quantity of state in the actual motor (8; 32) is input into the motor model (18; 46; 84) as the first quantity of state, an output of the amplifier (22; 52) is input into the motor model (18; 46; 84) as the second quantity of state, the motor (8; 32) is one of a DC motor, a DC brushless motor, and a series wound commutator motor, and the first quantity of state is a voltage applied to the motor (8; 32), the second quantity of state is a torque acting on the motor (8; 32), and the third quantity of state is a current flowing through the motor (8; 32), characterized in that the motor is configured to be installed in an electric tool provided with a tool unit and a power transmission unit for transmitting a rotation of the motor to the tool unit, the apparatus further comprises a power transmission unit model that is adapted to reflect a characteristic of the power transmission unit and has a torque acting on the power transmission unit from the tool unit as an input and a torque acting on the motor from the power transmission unit as an output, the output of the amplifier is input into the motor model via the power transmission unit model as the torque acting on the motor, and the apparatus is adapted to output the output of the amplifier as an estimated value of a torque acting on an actual tool unit. a motor model (18; 46; 84) that is adapted to reflect a characteristic of a motor (8; 32) and includes at least inputs pertaining to a first quantity of state and a second quantity of state relating to the motor (8; 32) and an output pertaining to a third quantity of state relating to the motor (8; 32); a comparator (20; 48) that is adapted to calculate a difference between the third quantity of state in an actual motor (8; 32) and the third quantity of state output from the motor model (18; 46; 84); and an amplifier (22; 52) that is adapted to amplify an output of the comparator (20; 48) at a predetermined gain, wherein the first quantity of state in the actual motor (8; 32) is input into the motor model (18; 46; 84) as the first quantity of state, an output of the amplifier (22; 52) is input into the motor model (18; 46; 84) as the second quantity of state, the motor (8; 32) is one of a DC motor, a DC brushless motor, and a series wound commutator motor, and the first quantity of state is a voltage applied to the motor (8; 32), the second quantity of state is a torque acting on the motor (8; 32), and the third quantity of state is a current flowing through the motor (8; 32), characterized in that the motor is configured to be installed in an electric tool provided with a tool unit and a power transmission unit for transmitting a rotation of the motor to the tool unit, the apparatus further comprises a power transmission unit model that is adapted to reflect a characteristic of the power transmission unit and has a torque acting on the power transmission unit from the tool unit as an input and a torque acting on the motor from the power transmission unit as an output, the output of the amplifier is input into the motor model via the power transmission unit model as the torque acting on the motor, and the apparatus is adapted to output the output of the amplifier as an estimated value of a torque acting on an actual tool unit.