An electric power steering apparatus comprising: an electric motor configured to generate a drive force to be given to an operation mechanism of an in-vehicle apparatus; a robust control unit configured to drive-control the electric motor comprising main and alternative functional parts; the main functional part having a first function that is a function relating to the drive-control of the electric motor; the alternative functional part having a second function that is an alternative function of the first function; a failure diagnosis part (121, 122) configured to diagnose a failure of the first control unit comprising a main function diagnosis part and an alternative function diagnosis part; characterized by a failure monitoring part configured to monitor an occurrence of a failure of the failure diagnosis part (121, 122); and an operation stop processing part configured to carry out a process of stopping an operation of the electric power steering apparatus when it is determined that the failure occurs in the failure diagnosis part (121, 122) based on a monitoring result of the failure monitoring part, wherein the main functional part and the alternative functional part are configured to be capable of operating in a first operation guaranteed range, the failure diagnosis part (121, 122) is configured to be capable of operating in a second operation guaranteed range including the first operation guaranteed range and being wider than the first operation guaranteed range, the failure monitoring part is configured to be capable of operating in a third operation guaranteed range including the second operation guaranteed range and being wider than the second operation guaranteed range, the failure diagnosis part (121, 122) is configured to diagnose whether the main functional part normally operates in the first operation guaranteed range, the failure monitoring part is configured to diagnose whether the failure diagnosis part (121, 122) normally operates in the second operation guaranteed range, the robust control unit includes a first control process that is a control process based on an output value of the main functional part and a second control process that is a control process based on an output value of the alternative functional part, the robust control unit is configured to perform the first control process to drive-control the electric motor when it is determined that the main functional part is normal based on a diagnosis result of the failure diagnosis part (121, 122), and the robust control unit is configured to perform the second control process to drive-control the electric motor when it is determined that the main functional part has a failure. an electric motor configured to generate a drive force to be given to an operation mechanism of an in-vehicle apparatus; a robust control unit configured to drive-control the electric motor comprising main and alternative functional parts; the main functional part having a first function that is a function relating to the drive-control of the electric motor; the alternative functional part having a second function that is an alternative function of the first function; a failure diagnosis part (121, 122) configured to diagnose a failure of the first control unit comprising a main function diagnosis part and an alternative function diagnosis part; characterized by a failure monitoring part configured to monitor an occurrence of a failure of the failure diagnosis part (121, 122); and an operation stop processing part configured to carry out a process of stopping an operation of the electric power steering apparatus when it is determined that the failure occurs in the failure diagnosis part (121, 122) based on a monitoring result of the failure monitoring part, wherein the main functional part and the alternative functional part are configured to be capable of operating in a first operation guaranteed range, the failure diagnosis part (121, 122) is configured to be capable of operating in a second operation guaranteed range including the first operation guaranteed range and being wider than the first operation guaranteed range, the failure monitoring part is configured to be capable of operating in a third operation guaranteed range including the second operation guaranteed range and being wider than the second operation guaranteed range, the failure diagnosis part (121, 122) is configured to diagnose whether the main functional part normally operates in the first operation guaranteed range, the failure monitoring part is configured to diagnose whether the failure diagnosis part (121, 122) normally operates in the second operation guaranteed range, the robust control unit includes a first control process that is a control process based on an output value of the main functional part and a second control process that is a control process based on an output value of the alternative functional part, the robust control unit is configured to perform the first control process to drive-control the electric motor when it is determined that the main functional part is normal based on a diagnosis result of the failure diagnosis part (121, 122), and the robust control unit is configured to perform the second control process to drive-control the electric motor when it is determined that the main functional part has a failure.