A lubricator comprising: a pump body configured to include: a cylinder configured such that one end thereof is closed and a remaining end thereof is open; an inlet passage formed on an upper portion of the cylinder perpendicularly to the cylinder, and formed to communicate with the cylinder; a first outlet passage formed on a lower portion of the cylinder, and formed in a location facing the inlet passage, and formed to communicate with the cylinder; and further including a second outlet passage formed on a lower portion of the cylinder, and spaced apart from locations, at which the inlet passage and the first outlet passage are formed, by predetermined distances in a direction of the remaining end of the cylinder, and formed to communicate with the cylinder; a piston configured to be inserted into the remaining end of the cylinder and reciprocating to the one end of the cylinder; first to fourth check valves (310, 320, 330, 340) provided on an inlet side of the inlet passage, outlet sides of the first and second outlet passages (130, 140), and the inner passage of the piston, respectively; a lubricant containing (410, 420, 430, 440, 450) unit configured to contain lubricant, and connected to the first check valve; and a piston driving unit (510, 520, 530, 540) configured to drive the piston so that an inlet port of the piston reciprocates between the one end of the cylinder and a location between the first and second outlet passages (130, 140); characterised in that the piston is configured to include: the inlet port formed at one end of the piston in direction of the end of the cylinder and to allow the lubricant to enter; a circumferential groove of a predetermined width formed on an outer circumference of a location spaced apart from the one end of the piston by a predetermined distance in the direction of remaining end of the piston; an inner passage formed to make the inlet port and the circumferential groove communicate with each other; wherein when the piston advances a predetermined distance in the direction of the one end of the cylinder, the piston closes the second outlet passage, and the first outlet passage is opened and the lubricant flows into the first outlet passage, when the piston further advances a predetermined distance in the direction of the one end of the cylinder and completely advances in the direction of the one end of the cylinder through the first outlet passage of the cylinder, the piston closes the first outlet passage and opens the second outlet passage, thus the lubricant flows into the second outlet passage through the inlet port, the inner passage and the circumferential groove of the piston, wherein the first outlet passage and the second outlet passage are selectively opened and closed only. a pump body configured to include: a cylinder configured such that one end thereof is closed and a remaining end thereof is open; an inlet passage formed on an upper portion of the cylinder perpendicularly to the cylinder, and formed to communicate with the cylinder; a first outlet passage formed on a lower portion of the cylinder, and formed in a location facing the inlet passage, and formed to communicate with the cylinder; and further including a second outlet passage formed on a lower portion of the cylinder, and spaced apart from locations, at which the inlet passage and the first outlet passage are formed, by predetermined distances in a direction of the remaining end of the cylinder, and formed to communicate with the cylinder; a piston configured to be inserted into the remaining end of the cylinder and reciprocating to the one end of the cylinder; first to fourth check valves (310, 320, 330, 340) provided on an inlet side of the inlet passage, outlet sides of the first and second outlet passages (130, 140), and the inner passage of the piston, respectively; a lubricant containing (410, 420, 430, 440, 450) unit configured to contain lubricant, and connected to the first check valve; and a piston driving unit (510, 520, 530, 540) configured to drive the piston so that an inlet port of the piston reciprocates between the one end of the cylinder and a location between the first and second outlet passages (130, 140); characterised in that the piston is configured to include: the inlet port formed at one end of the piston in direction of the end of the cylinder and to allow the lubricant to enter; a circumferential groove of a predetermined width formed on an outer circumference of a location spaced apart from the one end of the piston by a predetermined distance in the direction of remaining end of the piston; an inner passage formed to make the inlet port and the circumferential groove communicate with each other; wherein when the piston advances a predetermined distance in the direction of the one end of the cylinder, the piston closes the second outlet passage, and the first outlet passage is opened and the lubricant flows into the first outlet passage, when the piston further advances a predetermined distance in the direction of the one end of the cylinder and completely advances in the direction of the one end of the cylinder through the first outlet passage of the cylinder, the piston closes the first outlet passage and opens the second outlet passage, thus the lubricant flows into the second outlet passage through the inlet port, the inner passage and the circumferential groove of the piston, wherein the first outlet passage and the second outlet passage are selectively opened and closed only.