The spokesman did not say which of the units was shut. It has a 100,000 barrel per day (bpd) CDU and a 155,000 bpd CDU.
Idemitsu also shut the 190,000 bpd CDU at its Chiba refinery, east of Tokyo, on April 28 for regular maintenance, the spokesman said.
He did not indicate when the CDUs will resume operation but said it usually takes one and a half to two months to complete regular maintenance that is required every four years.
(Reporting by Yuka Obayashi; Editing by David Goodman)