PURPOSE:To shorten the measuring time required and detect an instantaneous measuring value as well as to aim at the safety of discharge operation, by making a necessary flow rate branch off from the downstream side of a control valve to an O2 sensor. CONSTITUTION:Exhaust gas is branched off from the downstream side of a leading pipe 7 and led outside, that is, discharged to the air via a filter 27, a cock 28 and a control valve 29. An O2 sensor 31 is connected to the downstream side of the control valve 29 via a pump 30 while large flow exhaust gas running inside the leading pipe 7 is decreased down to an almost constant flow rate by the control valve 29, then a necessary flow rate is fed to the O2 sensor 31 in addition. Thus, oxygen content in the exhaust gas fed to a discharge part 4 by way of the leading pipe 7 is measured every moment by a detector 23. With this constitution, a fixed quantity of the exhaust gas decompressed by the control valve 29 flows in the upstream side of the O2 sensor 31 so that time for arrival at the O2 sensor 31 can be sharply shortened.