A processor measures a pulse interval for a plurality of rotational phases of a rotary encoder, respectively. Each time the pulse interval is measured, the processor accumulates a measured value, which is the pulse interval or a moving average of the pulse interval of time-series data in association with corresponding one of the plurality of rotational phases, and records the measured value in a storage device. The processor derives a plurality of reference values corresponding to the plurality of rotational phases. Each time the measured value is newly obtained, the processor derives a correction value of a time interval of ink ejection of the inkjet recording apparatus in accordance with a difference between the measured value that is newly obtained and a corresponding reference value. The corresponding reference value corresponds to one of the rotational phases which corresponds to the measured value that is newly obtained.