Motor controller and construction machine provided therewith

There is obtained a configuration capable of rapidly decelerating a motor even if a failure occurs in the communication between a controller and a motor control unit in a motor controller in which a speed command is input from the controller to the motor control unit. A motor controller 1 is provided with a main controller 2, and a motor control unit 3 configured to be communicable with the main controller 2 to drive a motor M according to a torque command output from the main controller 2. The motor control unit 3 includes a speed control unit 31 for outputting a torque command, so as to decelerate the motor M, if any failure occurs in communication with the main controller 2 while the motor M is being driven.