A motorcycle comprising an exhaust device having a pipe section extending below an engine for guiding an exhaust gas therefrom rearwardly with respect to a vehicle body and housing a catalyzer therein, and a muffler connected to a rear end of said pipe section, comprising: an upstream oxygen sensor disposed on said pipe section and positioned upstream of said catalyzer, wherein said pipe section includes a larger-diameter portion having an increased diameter for housing said catalyzer therein, said upstream oxygen sensor is positioned upwardly of a central line (Oc) of said larger-diameter portion with respect to the vehicle body as viewed in side elevation of the vehicle body, said engine has a cylinder whose cylinder axis (Os) is inclined forwardly of the vehicle body with respect to a vertical direction, said larger-diameter portion is disposed such that said larger-diameter portion has an axis (Oc) substantially parallel to said cylinder axis (Os), said upstream oxygen sensor is mounted outwardly from the inside of said pipe section in the vehicle widthwise direction, said larger-diameter portion is positioned outside of an oil filter which is projected into a front surface of a crankcase, and an intermediate pipe coupled to the rear end of the larger-diameter portion passes through side portions of an oil pan below said crankcase; characterised in that the exhaust device further comprises: a downstream oxygen sensor disposed on said pipe section and positioned downstream of said catalyzer, wherein said downstream oxygen sensor is positioned upwardly of a central line (Oc) of said larger-diameter portion with respect to the vehicle body as viewed in side elevation of the vehicle body, and said downstream oxygen sensor is disposed in said larger-diameter portion. an upstream oxygen sensor disposed on said pipe section and positioned upstream of said catalyzer, wherein said pipe section includes a larger-diameter portion having an increased diameter for housing said catalyzer therein, said upstream oxygen sensor is positioned upwardly of a central line (Oc) of said larger-diameter portion with respect to the vehicle body as viewed in side elevation of the vehicle body, said engine has a cylinder whose cylinder axis (Os) is inclined forwardly of the vehicle body with respect to a vertical direction, said larger-diameter portion is disposed such that said larger-diameter portion has an axis (Oc) substantially parallel to said cylinder axis (Os), said upstream oxygen sensor is mounted outwardly from the inside of said pipe section in the vehicle widthwise direction, said larger-diameter portion is positioned outside of an oil filter which is projected into a front surface of a crankcase, and an intermediate pipe coupled to the rear end of the larger-diameter portion passes through side portions of an oil pan below said crankcase; characterised in that the exhaust device further comprises: a downstream oxygen sensor disposed on said pipe section and positioned downstream of said catalyzer, wherein said downstream oxygen sensor is positioned upwardly of a central line (Oc) of said larger-diameter portion with respect to the vehicle body as viewed in side elevation of the vehicle body, and said downstream oxygen sensor is disposed in said larger-diameter portion.