A noise filter comprising: an active filter circuit connectable to: i) a power-receiving terminal (20c, 20d) of a power line for: a) an alternating-current power supplied to a power converter device from an alternating-current power grid; or b) a direct-current power source interconnected with the alternating-current power grid, or ii) a power-receiving terminal (20c, 20d) of a power line for a direct-current power supplied from the direct-current power source; the active filter circuit being configured to reduce a harmonic component of a conduction noise propagating to the power line and to output the reduced harmonic component to the power line; a controller configured to monitor a variation in a state of an input power entering a power source module (11, 40) for generating drive power for an active element constituting the active filter circuit, or a variation in a state of the drive power supplied from the power source module (11, 40), and to diagnose an abnormality of a circuit operation for a circuit including the active element of the active filter circuit therein on the basis of a variation in the state of the input power or the variation in the state of the drive power, wherein the power source module i (11, 40) includes a positive power source configured to generate a positive-side voltage and a negative power source configured to generate a negative-side voltage, wherein the controller is configured to monitor the variation in the state of the drive power supplied from the power source module on the basis of at least any one current of a first current flowing from the positive power source of the power source module to a reference potential, a second current flowing from the negative power source in the power source module to the reference potential, and a third current flowing from a connection point for connecting a negative-side electrode of the positive power source and a positive-side electrode of the negative power source to the reference potential. an active filter circuit connectable to: i) a power-receiving terminal (20c, 20d) of a power line for: a) an alternating-current power supplied to a power converter device from an alternating-current power grid; or b) a direct-current power source interconnected with the alternating-current power grid, or ii) a power-receiving terminal (20c, 20d) of a power line for a direct-current power supplied from the direct-current power source; i) a power-receiving terminal (20c, 20d) of a power line for: a) an alternating-current power supplied to a power converter device from an alternating-current power grid; or b) a direct-current power source interconnected with the alternating-current power grid, or a) an alternating-current power supplied to a power converter device from an alternating-current power grid; or b) a direct-current power source interconnected with the alternating-current power grid, or ii) a power-receiving terminal (20c, 20d) of a power line for a direct-current power supplied from the direct-current power source; the active filter circuit being configured to reduce a harmonic component of a conduction noise propagating to the power line and to output the reduced harmonic component to the power line; a controller configured to monitor a variation in a state of an input power entering a power source module (11, 40) for generating drive power for an active element constituting the active filter circuit, or a variation in a state of the drive power supplied from the power source module (11, 40), and to diagnose an abnormality of a circuit operation for a circuit including the active element of the active filter circuit therein on the basis of a variation in the state of the input power or the variation in the state of the drive power, wherein the power source module i (11, 40) includes a positive power source configured to generate a positive-side voltage and a negative power source configured to generate a negative-side voltage, wherein the controller is configured to monitor the variation in the state of the drive power supplied from the power source module on the basis of at least any one current of a first current flowing from the positive power source of the power source module to a reference potential, a second current flowing from the negative power source in the power source module to the reference potential, and a third current flowing from a connection point for connecting a negative-side electrode of the positive power source and a positive-side electrode of the negative power source to the reference potential.