An apparatus for analyzing sulfur components, including sulfur dioxide, in a sample gas containing nitrogen monoxide, using ultraviolet fluorescence, comprising: an introducing pipe for introducing a sample gas, an ultraviolet fluorometric analyzer for detecting said sulfur components and connected to said introducing pipe, a pretreatment section provided upstream of said fluorometric analyzer, said pretreatment section provided for oxidizing said sulfur components other than sulfur dioxide in said sample gas to sulfur dioxide, and means (5, 25, 6) for oxidizing nitrogen monoxide to nitrogen dioxide before detecting said sulfur components, including a supply pipe for oxidizing gas and being installed upstream of said pretreatment section in said introducing pipe downstream from an inlet end thereof or immediate after the introducing pipe, and being configured such that an oxidation gas is added and mixed into the sample gas while controlling the flow rate of the oxidation gas and the flow rate of the sample gas at a constant ratio at all times, means (4, 7, 8, 25) provided for controlling the flow rates of the sample gas and of the oxidation gas at said constant ratio including a flow rate adjusting valve capable of adjusting the flow rate of the sample gas, a flow rate adjusting valve capable of adjusting the flow rate of the oxidation gas, and a by-pass pipe provided for discharging excessive sample gas to the outside and connected to and as branch of the introducing pipe, said by-pass pipe being provided with an automatic control valve arranged to detect the flow rate in said supply pipe and the opening degree of said automatic control valve being adjustable based on the detected flow rate in said supply pipe. an introducing pipe for introducing a sample gas, an ultraviolet fluorometric analyzer for detecting said sulfur components and connected to said introducing pipe, a pretreatment section provided upstream of said fluorometric analyzer, said pretreatment section provided for oxidizing said sulfur components other than sulfur dioxide in said sample gas to sulfur dioxide, and means (5, 25, 6) for oxidizing nitrogen monoxide to nitrogen dioxide before detecting said sulfur components, including a supply pipe for oxidizing gas and being installed upstream of said pretreatment section in said introducing pipe downstream from an inlet end thereof or immediate after the introducing pipe, and being configured such that an oxidation gas is added and mixed into the sample gas while controlling the flow rate of the oxidation gas and the flow rate of the sample gas at a constant ratio at all times, means (4, 7, 8, 25) provided for controlling the flow rates of the sample gas and of the oxidation gas at said constant ratio including a flow rate adjusting valve capable of adjusting the flow rate of the sample gas, a flow rate adjusting valve capable of adjusting the flow rate of the oxidation gas, and a by-pass pipe provided for discharging excessive sample gas to the outside and connected to and as branch of the introducing pipe, said by-pass pipe being provided with an automatic control valve arranged to detect the flow rate in said supply pipe and the opening degree of said automatic control valve being adjustable based on the detected flow rate in said supply pipe.