A numerical controller and a servomotor control system, capable of axis control required in case of a communication failure in the control of servomotors driven by servo amplifiers connected to a plurality of serial buses. The numerical controller has a plurality of serial buses to which the numerical controller and a plurality of servo amplifiers are daisy-chain-connected and controls servomotors connected individually to the servo amplifiers. The device comprises a plurality of communication control circuits for detecting respective communication failures of the serial buses and selecting means for selecting whether or not to notify the other communication control circuits of a communication failure, if any, of any one of the communication control circuits detected thereby. In the control of the servomotors driven by the servo amplifiers connected to the serial buses, an output of a servo amplifier connected to a serial bus that suffers a communication failure is reduced to 0 so that outputs to the servo amplifiers connected to the remaining normal serial buses can be maintained.