Reuters quotes IJU deputy head Mashallah Shamsolvaezin as saying union member Ali Farahbakhsh is being held in Tehran's Evin prison.
Shamsolvaezin called on Iranian authorities to announce immediately the reason for his arrest.
ISNA quoted Sohrab Soleimani, the director of Tehran prisons organizations, as saying Farahbakhsh
was in jail and in good health.
Iranian authorities keeps a tight rein on the country's press and have shut down many newspapers and detained dozens of reporters in recent years.