A Japanese judge on temporary assignment with the Financial Services Agency is being investigated by the securities watchdog on suspicion of insider trading, sources familiar with the matter said Saturday. The Securities and Exchange Surveillance Commission is analyzing transactions with an eye to filing a criminal complaint with Tokyo prosecutors, they said. The judge in his 30s, who was in charge of reviewing documents submitted by companies planning tender offers, came under scrutiny in September after the watchdog detected a suspicious transaction, according to the sources. The FSA said it…