The function of a steel open‐type dam is to trap debris flow. To fulfill this function, the member spacing of a permeable steel open‐type dam should be set to one to two times the maximum or the most boulder diameter. The maximum or the most boulder diameter is determined by cumulative curve using the boulder that have the potential to become debris flow from that location, with the 95th percentile boulder diameter being the maximum boulder diameter and the maximum value in the most boulder diameter zone being the most boulder diameter. In this survey, 200 boulders are selected by the measurer from the available boulder in the field. For this reason, arbitrariness cannot be completely excluded from the fact that the measurer arbitrarily selects boulder. This means that the boulder diameter distribution obtained by the boulder diameter survey is different from the actual boulder diameter distribution. We investigated the distribution of boulder diameter close to the total number using UAV so as not to omit the boulder that was previously omitted by the measurer. As a result, it was confirmed that the maximum and most boulder diameter obtained by the random method contributed to the blockage of the member spacing at the opening of the permeable steel open‐type dam.