The man, an employee of the western Japanese city of Kinokawa, visited porn sites from June 2007 to February 2008, city official Tomiko Waki said. Waki said the man logged about 170,000 hits on adult sites in July 2007 alone.
The employee's name was withheld.
According to city officials, a single click on many types of pornographic websites can register multiple hits on a computer's internal log.
The demoted employee's co-workers reportedly were unaware of his porn viewing, which took place throughout the workday.
"Each desk is set apart from each other," Waki said.
A virus infection prompted the employee's supervisors to investigate his Web browser's history. Along with the demotion, he received a 20,000 yen ($190) monthly pay cut, Waki said.