The present invention relates to particle measuring apparatus and a particle measuring method, whereby particle counts per size range can be measured with a high accuracy using a laser power scanning in which lasers of several powers are sequentially irradiated. First, minimum powered lasers capable of measuring particles having more than relevant size are irradiated to a particle measurement space for a predetermined time in response to plural counts of each mutually different particle size, and the counts of particles per size are measured by detecting a scattered light. Furthermore, the counts of particles belonging to each size range can be accurately calculated through an algorithm using the actually measured value.