Former Sumitomo Corp. copper trader Yasuo Hamanaka pleaded guilty to covering up massive trading losses that depressed world copper prices and battered the image of his 300-year-old company. The former star trader, who was arrested Oct. 22 and charged with forging signatures and swindling a Sumitomo subsidiary out of $770 million, entered the plea on the first day of his trial in Tokyo District Court. Prosecutors alleged Hamanaka made the fraudulent transactions to cover up his massive losses in copper trading, estimated by Sumitomo at $2.6 billion. The scandal, which broke in June, depressed world copper prices and called into doubt the management practices at one of the country's most powerful companies. Hamanaka faces a sentence of up to five years on the forgery charge and up to 10 years on the fraud charge.