An antifriction bearing includes a fixed ring which comprises a steel containing up to about 10% of residual austenite. A first surface of the light guide array is a plane evenly distanced from the LED array and a second surface of the light guide array is a curved surface evenly distanced from the curved housing. As the heat-absorbing area has a heat capacity larger than that in the other portions, the combustion heat is absorbed and dispersed even if the rapid chain reaction of PM combustion occurs, whereby it is possible to suppress the temperature rise of the downstream portion of the particulate filter liable to be high in temperature.