Usually, we measure particulate matter weight using PM sensor in production. The PM sensor needs measurement period of 30 seconds due to whole gas flow for weight accumulation. This paper presents a study on measurement of particulate matter weight by transmitted light attenuation method based on Lambert-Beer law. We propose that the intensity of transmitted light attenuation is proportional to the particulate matter weight. Our transmitted light attenuation method clarifies the transition of Soot weight and enables whole weight measurement within 400 milliseconds by LASER density attenuation through PM gas. In addition, we introduce the effects for light attenuation by PM diameter distribution.