Researches on the marine algal flora along the southwestern coast of the San-in District of Honshu which extends from Omishima, Yamaguchi Pref., to Yodo-e, Tottori Pref., were carried out by the writer during a period ranging from August 1966 to June 1976. This coast, which consists of rocky beaches and sandy ones mixed nearly half and half, is exposed to violent waves during typhoon and monsoon seasons of the year. As a result of the researches, the writer could collect many kinds of marine plants including 20 spp. of Cyanophyceae belonging to 5 families and 11 genera, 33 spp. of Chlorophyceae belonging to 9 families and 11 genera, 57 spp. of Phaeophyceae belonging to 17 families and 36 genera, 106 spp. of Rhodophyceae belonging to 25 families and 58 genera, and 3 spp. of Sea-grasses belonging to 1 family and 2 genera. The species marked with ○ in the list, 36 in number are those newly added to the flora of the San-in District as a whole, which extends from Hikoshima Islet, Shimonoseki City in Yamaguchi Pref., to Shomenzaki Point, Takahama-cho in Fukui Pref.. Among these species, the following 15 species are also new to the Japan Sea coast of Honshu : Xenococcus acervatus, Hydrocoleum coccineum, Phormidium submembranaceum, Spirulina tenerrima, Oscillatoria ampphibia, Monostroma latissimum, Cladophora ohkuboana, Chaetomorpha spiralis, Caulerpa peltata var. peltata, Dictyota dilatata, Distromium decumbens, Erythrotrichia biseriata, Bangia ciliaris, Colaconema simplex, and Acrochaetium, crassipes. And 2 species, Ulva fasciata and Homoeostrichus flabellatus, are new to the Japan Sea. The southwestern coast of the San-in District seems not only to be the southern limit of distribution in the Japan Sea for Xenococcus cladophorae and Plocamium recurvatum, but also the eastern limit for Bryopsis maxima, Derbesia tenuissima, Codium coarctatum, Codium cylindricum, Codium mamillosum, Ecklonia cava and Galaxaura apiculata.