Journalist Kenji Goto converted to Christianity in 1997 and has spoken about his faith extensively in the past.
Japanese broadcasters have said they've been informed a statement from IS will be released soon.
A militant had appeared in a video with Mr Goto and another Japanese man, Haruna Yukawa, and threatened to kill them unless a 200 million US dollars ransom was paid.
The jihadist said the Japanese government had 72 hours to make the payment but ministers in the country said it would never negotiate with terrorists.
Mr Goto's mother has made an impassioned plea for him to be released.
Junko Ishido said: "My son is not an enemy of the Islamic State".
She said her son had gone to Syria to try and secure a friends release.
"Ever since before he learned to walk, my son has been kind to all of the children he knew.
"My son felt he had to do everything in his power to try to rescue a friend and acquaintance."
Speaking about his faith to a Christian magazine in Japan the journalist said: "I have seen horrible places and have risked my life, but I know that somehow God will always save me."
But he added he would never deliberately do anything dangerous, "Do not put the Lord your God to the test," he said.