A cooling fan controller that is configured to be connected to a host controller via a first signal line and a second signal line and that is configured to control rotation of a cooling fan on the basis of a rotation command output from the host controller via the first signal line, wherein the cooling fan controller comprises: an abnormality detector configured to detect an abnormality of a monitoring target in a mechanism configured to control the cooling fan; characterized in that the cooling fan controller further comprises: a storage storing a table in which a rotational speed of the cooling fan is related to a duty ratio corresponding to the rotational speed of the cooling fan; a signal processor configured to acquire a rotational speed of the cooling fan and to generate a signal of a duty ratio corresponding to the acquired rotational speed with reference to the table; and an output part configured to: - output a first signal to the second signal line when the abnormality is not detected by the abnormality detector, said first signal corresponding to the signal generated by the signal processor; and - output a second signal to the second signal line when the abnormality is detected by the abnormality detector, said second signal being different from the first signal. an abnormality detector configured to detect an abnormality of a monitoring target in a mechanism configured to control the cooling fan; characterized in that the cooling fan controller further comprises: a storage storing a table in which a rotational speed of the cooling fan is related to a duty ratio corresponding to the rotational speed of the cooling fan; a signal processor configured to acquire a rotational speed of the cooling fan and to generate a signal of a duty ratio corresponding to the acquired rotational speed with reference to the table; and an output part configured to: - output a first signal to the second signal line when the abnormality is not detected by the abnormality detector, said first signal corresponding to the signal generated by the signal processor; and - output a second signal to the second signal line when the abnormality is detected by the abnormality detector, said second signal being different from the first signal. a storage storing a table in which a rotational speed of the cooling fan is related to a duty ratio corresponding to the rotational speed of the cooling fan; a signal processor configured to acquire a rotational speed of the cooling fan and to generate a signal of a duty ratio corresponding to the acquired rotational speed with reference to the table; and an output part configured to: - output a first signal to the second signal line when the abnormality is not detected by the abnormality detector, said first signal corresponding to the signal generated by the signal processor; and - output a second signal to the second signal line when the abnormality is detected by the abnormality detector, said second signal being different from the first signal. - output a first signal to the second signal line when the abnormality is not detected by the abnormality detector, said first signal corresponding to the signal generated by the signal processor; and - output a second signal to the second signal line when the abnormality is detected by the abnormality detector, said second signal being different from the first signal.