The relationship between chlorophyll a concentration at the sea surface and standing stock of chlorophyll a in the euphotic layer in the Seto Inland Sea was investigated in order to estimate the primary production from the data of chlorophyll a concentration at the sea surface obtained from the ocean color image by a satellite. We may reasonably conclude that the standing stock of chlorophyll a in the euphotic layer and primary production can be estimated from the chlorophyll a concentration at the sea surface in the Sew Inland Sea.