In early to late spring, spawning shoals of a squid, Lorigo bleekeri, migrate to the western coast of Hooked facing the Japan Sea. Breakwaters and other artificial structures of the harbors are suspected to be utilized for their spawning. Diving observations were conducted of breakwaters in nine harbors to confirm actually laid eggs and to characterize underwater conditions of the spawning sites. spatial features of these spawning sites and luminous condition were described. The selected depths and distribution of the spawning sites observed to be affected by waves, local currents and temperatures. Function of unexpected role of breakwaters as spawning sites of a squid, improvements of the breakwater and other seaside structures were suggested.