An output measurement system of a large-sized low-speed two-stroke engine that measures output of an engine for one cycle, the system comprising: a pressure sensor unit that detects individual combustion pressure of cylinders; an angle sensor unit that detects rotation angles of a crankshaft with a Z-pulse thereof matched with an actual TDC of a predetermined reference cylinder; and an output measurer that collects combustion pressure of the cylinders at the rotation angles of the crankshaft from the pressure sensor unit and the angle sensor unit, shows a plurality of graphs about combustion pressure of the cylinders at the rotation angles of the crankshaft, combustion pressure of the cylinders at combustion chamber volumes, and pressure change rates of the cylinders at the rotation angles of the crankshaft, finds out the position of a compression TDC and a loss of angle of the reference cylinder from the graph about the pressure change rates of the cylinders at the rotation angles of the crankshaft, adjusts points in time when the angle sensor unit detects the rotation angle of the crankshaft for the other cylinders except for the reference cylinder, and then measures output of the cylinders, characterized in that the output measurer matches the positions of compression TDCs of the other cylinders except for the reference cylinder with the position of the compression TDC of the reference cylinder in the graph about the pressure change rates of the cylinders at the rotation angles of the crankshaft by adjusting the points in time when the angle sensor unit detects the rotation angle of the crankshaft for the other cylinders except for the reference cylinder. a pressure sensor unit that detects individual combustion pressure of cylinders; an angle sensor unit that detects rotation angles of a crankshaft with a Z-pulse thereof matched with an actual TDC of a predetermined reference cylinder; and an output measurer that collects combustion pressure of the cylinders at the rotation angles of the crankshaft from the pressure sensor unit and the angle sensor unit, shows a plurality of graphs about combustion pressure of the cylinders at the rotation angles of the crankshaft, combustion pressure of the cylinders at combustion chamber volumes, and pressure change rates of the cylinders at the rotation angles of the crankshaft, finds out the position of a compression TDC and a loss of angle of the reference cylinder from the graph about the pressure change rates of the cylinders at the rotation angles of the crankshaft, adjusts points in time when the angle sensor unit detects the rotation angle of the crankshaft for the other cylinders except for the reference cylinder, and then measures output of the cylinders, characterized in that the output measurer matches the positions of compression TDCs of the other cylinders except for the reference cylinder with the position of the compression TDC of the reference cylinder in the graph about the pressure change rates of the cylinders at the rotation angles of the crankshaft by adjusting the points in time when the angle sensor unit detects the rotation angle of the crankshaft for the other cylinders except for the reference cylinder.