Oman's Sultan Qaboos will travel to Belgium on Saturday for unspecified medical checks, his court said.
Qaboos, 79, has ruled the Arabian peninsula state since he took over in 1970. He has travelled abroad for medical reasons at least twice since 2014.
The court statement said the latest checks should take "a limited period of time" but gave no details on his condition.
Qaboos has no children and has not publicly appointed a successor.
*This story had been edited by Ahram Online