A lubricating device includes an lubricant impregnation member to coat a surface of a target member with lubricant, an lubricant supply member to supply lubricant to the lubricant impregnation member, a subsidiary roller to feed the lubricant impregnation member out, a main roller to reel in the lubricant impregnation member fed by the subsidiary roller, a drive motor to drive the main roller, an encoder disc provided at a rotational shaft of the subsidiary roller, an encoder sensor to detect a rotational state of the encoder disc, and a controller unit to calculate an outer diameter of the main roller and a portion of the lubricant impregnation member reeled by the main roller, based on a number of drive steps of the drive motor during a detection interval of the encoder disc.