SEOUL—Choi Kun-chol says he didn't know he had spent several years helping to fill Kim Jong Il's private slush fund until he left North Korea.

Like the thousands of others working under the North Korean government division known as Office 39, Mr. Choi was told by superiors that he was generating money to build a strong socialist economy. It was referred to as the "revolutionary fund," he said in his first media interview since defecting a few years ago.

The...