Abstract: An apparatus for deciding engine-oil replacing time for an automobile. The apparatus comprises a pulse input unit into which is inputted a pulse generated during the rotation of the engine of an automobile to calculate rotational frequency of the engine on the basis of the pulse, a rotational-frequency integration unit for receiving a signal of rotational frequency of the engine output from the pulse input unit to integrate the rotational frequency of the engine, and a display unit into which is input an integrated value from the rotational-frequency integration unit and in which a degree of oil degradation on the basis of the integrated value is displayed by a display such as a LED, a liquid crystal, a 7-segment or the like.