A cooling fan controller that is configured to be connected to a host controller via a first signal line (L1) 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 (L1), 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 (L2) 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 (L2) 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 (L2) 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 (L2) 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 (L2) 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 (L2) 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 (L2) 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 (L2) when the abnormality is detected by the abnormality detector , said second signal being different from the first signal. <EOS>