OSAKA -- Investigators believe an overseas counterfeiting ring is behind the recent discoveries of more than 50 fake 10,000 yen notes in Fukuoka, Osaka and Tokyo, informed sources said.

Police are on full alert for more counterfeit bills, which are likely to be used in other areas around the country. An investigation is being carried out to find those who used the fake currency, they said.

According to investigators, all the fake bills were printed in a series, and with corresponding serial numbers. They also bore a redder tinge compared with the "darker" color of fake notes made previously by Japanese counterfeiting rings.

Use of the fake notes by several men said to range in age from 20 to 40 was witnessed in Osaka and Fukuoka, sources said. Police have learned that a young man who allegedly used a counterfeit 10,000 yen bill in Fukuoka was wearing a baseball cap similar to one another suspect in Osaka was seen wearing. They suspect the ring has moved from Fukuoka to Osaka.

One counterfeit 10,000 yen bill was reportedly used to pay a taxi driver, who told police the suspect could speak little Japanese, except to name his destination. A counterfeit expert said the fake bills are high-quality. "The lines and designs of the note are reproduced in detail. The counterfeit ring must have veterans who have been doing this for more than 20 years, in view of these reproduction skills," he said. But he added that the serial numbers were believed printed manually afterward, and that they were crooked. "Compared with sophisticated techniques, these unbalanced jobs lead me to think that the work is too childish for Japanese," he said.