The present invention provides an adsorbent comprising calcined coke with a BET surface area of 20 m.sup.2/g or less which is obtained by calcining a particular carbonaceous material at 1000 to 1500.degree. C. and which is preferably obtained by calcining coal coke, petroleum coke or charcoal coke at 1000 to 1500.degree. C. The adsorbent of the present invention exhibits adequate adsorbent ability with respect to an oil component in oil-containing wastewater even when the oil-containing wastewater is an emulsion or contains a low concentration of the oil component, and also stably maintains the adsorbent ability over a long period of time.