S1 No. 1 Style (エスワン ナンバーワンスタイル) or just S1 (エスワン) is a Japanese producer of adult videos (AV) which has its offices in Tokyo's Meguro Ward. The company is best known for retailing its adult videos under the Fanza brand, an affiliate of DMM. In the past 15 years since its inception, S1 has risen to become one of the most prominent movie studios in the Japanese adult entertainment scene, producing over 5,100 movies to date.
S1 is one of the many affiliated companies in the giant Hokuto Corporation group. Hokuto distributes the studio's videos via retail sales and online through its DMM website under the Fanza brand. The Public Relations representative for the studio is Megumi Yamane (山根愛美).
The studio's website, s1s1s1.com, has been online since June 30, 2004 and their first videos were released in November 2004 starring AV Idols Sora Aoi and Alice Ogura along with the simultaneous debut of eight new actresses: MEW, Mami Hayasaki, Kaya Yonekura, Chihiro Hara, Kanan Kawai, Akane Mochida, Rin Hino and Misato Shiraishi. All Japanese adult videos are censored by applying a mosaic over the genital areas but in late 2004, S1 was the first company to replace the old analog mosaics with a new thinner digital mosaic they named "girigiri" or risky mosaic (ギリギリモザイク, girigirimozaiku).
S1 released its first 11 videos on November 11, 2004 labeled with the production codes ONED-001 to ONED-011. The company continued using the ONED series of production codes, releasing as many as 30-40 videos per month, until ONED-999 appeared on June 19, 2008 when it switched to a new code series starting with SOE-001. Compilation videos which re-mix previously released material use production codes of the type starting with ONSD-001. In addition, S1 videos intended "for rental only" carry different codes. Original videos are labeled 5ONE-xxx or 5SOE-xxx and compilation videos follow the scheme 5ONS-xxx, where the "xxx" portion corresponds to the 3 digits of the same "for sale" videos.