A proposal is made for efficiently estimating conidial numbers of Japanese pear scab in rain water. The proposed concentration method consists of a few steps as follows; conidia are trapped in an agar solution by adding a small amount of diluted agar solution to conidial suspension, centrifuging them, and pouring supernatant water out. It was confirmed that seasonal changes in the number of conidia in rain water which flows down on a Japanese pear shoot were pursued with this method effectively and efficiently. Through this method, changes in the number of conidia at the primary infection source, formed on scale lesions were, also identified: i.e. scale lesions appeared in late January and bud lesions did before flowering.