A light receiving device for an optical encoder comprises: a plurality of first photodiodes juxtaposed in a first direction and extending in a second direction that is generally vertical to the first direction; and a light shielding layer. Natural gas liquids may be first separated and removed before fractionation. Since a light contact state can constantly be maintained in machining according to a monitoring condition including a lightheavy contact-determining threshold, cutting resistance will not increase, thereby preventing damage on the tool and deterioration in machining accuracy.