The video was released by the group on Wednesday.
Thousands of Islamists and suspected supporters of the outlawed Muslim Brotherhood group are in prison in Egypt, a majority Muslim country. They were imprisoned after 2013 military overthrow of Islamist President Mohammed Morsi.
The apparent hostage in the video identifies himself as Tomislav Salopek, reported CNN on Thursday.
Salopek who works for a French geoscience firm was seen kneeling in a jumpsuit in front of a masked, knife-holding man in camouflage.
He was kidnapped while heading to work on July 22 in a company car which belongs to French firm Compagnie Generale de Geophysique (CGG), where he worked for as a topographer.
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The Croatian government is "making all the efforts to resolve the difficult situation of the Croatian citizen T.S.," the Croatian Foreign Ministry said Wednesday, in an apparent reference to Salopek.