We investigated the changes in plasma sex steroid hormones, testosterone (T), estradiol-$17{eta}$ ($E_2$), 17,$20{eta}$-dihydroxy-4-pregnen-3-one ($17{alpha}20{eta}P$), 11-ketotestosterone (11KT) and cortisol levels from ribbed gunnel, Dictyosoma burgeri in associated with annual reproductive cycle. The gonadosomatic index (GSI) of females increased from November, peaked in February and decreased rapidly from March. The GSI of males also increased from November, peaked in January and then decreased gradually. In females, $E_2$ levels increased and remained high from December to February. The levels of T showed a similar tendency and correlated ($r_s$=0.898, p<0.01) with $E_2$ levels. The levels of $17{alpha}20{eta}P$ increased rapidly in February ($4.78{pm}1.01ng/ml$) and peaked in July ($5.08{pm}0.65ng/ml$). Cortisol level was peaked in March and correlated with $17{alpha}20{eta}P$ levels ($r_s$=0.696, p<0.01). In males, the levels of T was peaked in January and then decreased rapidly. The levels of 11KT were remained high from October to January. On the other hand, the levels of $17{alpha}20{eta}P$ fluctuated during reproductive cycle. These results suggest that plasma sex steroids in ribbed gunnels have annual periodicity, and that cortisol may involve in maturation of females.