The main target of boat seine fisheries in the western sea area of the Kii Channel is anchovy larvae, Engraulis japonica. The fishing gear consists of wings, body net, and codend. The gear is made of various minnow netting, and the mesh size of the codend is 260 kei: the number of twines in webbing 50cm wide. Five experimental hauls were carried out with small-mesh pocket-nets attached to several positions on the body net and the codend. The selectivity curve for anchovy larvae was estimated with an extended SELECT model for analyzing body size (total length) distributions of the pockets catch and sample from the codend catch. Selectivity parameters were varied in the five hauls, that is, l50 (the length of 50% retention) was 8.7-12.3mm and SR (selection range=l75−l25) ranged from 3.0mm to 4.7mm. Higher catch volumes were likely to prevent small fish from escaping. In this fishery, the codend catch weight usually exceeds 20kg, which means that the l50 is smaller than 10mm, and few anchovy larvae smaller than 15mm are distributed in this fishing ground. Therefore, the current codend mesh size, 260 kei is too fine, and consequently enlargement of the mesh size should be examined for fisheries management and product management.