In order to examine the error of particle concentration measured by non-suction sampling, theoretical and experimental studies have been conducted. Theoretical calculations under the assumption of viscous flow are in good agreement with the experimental result obtained by Levin. However, Levin's data were not affected by reentrainment of particles, because they were obtained by photographic method. In actual non-suction sampling, the particles are rebound and entrained by air flow. As a result, the collection efficiency has the largest value at the beginning of measurement, and decreases with the sampling time. It was observed that no particles deposited on the inside wall of the probe from the inlet to some distance, b, into the probe. The distance b was proportional to the dynamic pressure of air flow. The efficiencies at the beginning agreed well with the theoretical values which were modified in order to take the distance b into consideration.