In April tax authorities searched Rato's Madrid home and offices and ordered his accounts frozen over the latest allegations.
Rato, 66, was formerly a prominent member of Spain's governing Popular Party. He was Spain's finance minister in the late 1990s and managing director of the IMF from 2004 to 2007.
On Monday an investigating judge summoned him for questioning on July 22nd for a hearing to decide whether to bring formal charges against him, the courts service said in a statement.
The judge also heard evidence on Monday from the head of the National Fraud Investigation Office, Margarita Garcia-Valdecasas, as part of the inquiry, it added.
Rato was already being investigated for alleged fraud during his time as chief executive at Bankia, a Spanish lender which needed to be bailed out by the government in 2012.
The near-collapse of Bankia almost brought down Spain's whole financial sector, which was bailed out later that year by international creditors for €41 billion ($44 billion).
Rato has also been questioned in court as part of a third probe into alleged spending sprees on company credit cards by him and other ex-managers of Bankia.
He has denied wrongdoing in both those cases.
The latest probe launched in April centres on allegations of tax evasion, money-laundering and fraudulent conveyance for allegedly concealing his assets.