OSAKA -- With convenience like late-night or early morning service and never having to leave one's own home, stock trading via the Internet has not surprisingly become increasingly active.It is also alluring to securities companies, who are able to expand their business without opening new branches or hiring more staff.Imagawa Securities Co., a medium-size securities company based in Osaka, began trading shares on the Internet in January. Its home page initially had 320,000 visitors monthly, but that number nearly tripled to 900,000 in August.Investors can buy shares by keying stock codes and other data into their computers between the hours of 6 p.m. and 2 a.m. They can trade in stocks listed on the country's main exchanges, in over-the-counter stocks and in medium-term national bond funds.Payment is settled through bank accounts opened by investors in advance. Daiwa Securities Co., which introduced Internet stock trading in April 1996, said purchase orders are most active from 10 p.m. through 1 a.m.The average investor is a man in his 30s or 40s who is sensitive about his own responsibility, according to the securities company. Many Internet investors are salaried workers who are busy working during the day but trade in stocks from their study after returning home, they said.Internet stock trading is becoming a strategic pillar for securities companies. Imagawa Securities has six trading offices in the Kinki region but its clients are scattered over 45 prefectures from Hokkaido to Okinawa.Tadahiko Sugimura, president of Imagawa Securities, said Net trading is particularly beneficial to small and medium-size securities companies because it expands their businesses without new branches or personnel.In the United States, the leading nation in Internet securities trading, brokers who offer large discounts on commissions are on the increase. Furthermore, private trading stores are being opened on the Net to mediate in purchase and selling orders.