The Paris appeals court has ordered the Societe Generale trader Jerome Kerviel to be jailed while massive losses at the bank are being investigated.
The court ruled that he should be detained because of the "necessities of the investigation" and the risk that he could flee the country.
Earlier, an employee of the brokerage firm Fimat - owned by Societe Generale - was also detained by police.
The man was questioned about his links to Mr Kerviel.
Fimat was a division of Societe Generale, which was recently merged with another brokerage owned by French bank Credit Agricole and renamed Newedge.
A report in Le Monde said Mr Kerviel had conducted some of his trades through Fimat and that police suspect the brokerage employee may have been aware of his activities.
Mr Kerviel has been blamed for incurring massive losses, costing Societe Generale 4.9bn euros ($7bn; £3.7bn).
He was freed on 28 January following two days of questioning by magistrates.
Now he will be put in "provisional detention" while the case is being investigated.
Mr Kerviel is under investigation for breach of trust, computer abuse and falsification.