Florian Philippot said the tax would apply to non-nationals, including citizens of other European Union countries.
"Alternatively, he (an employer) can hire a French national and avoid having to pay the tax," Philippot told RTL radio.
Marine Le Pen, leader of the anti-immigrant, anti-European Union National Front party, is regularly tipped by pollsters to make the May 7 runoff of France's two-round presidential contest but lose that final duel.
Philippot also called on the conservative candidate Francois Fillon, engulfed by a scandal over public money he is accused of paying to his wife for work she did not do, to pull out of the race.
"I want Francois Fillon to live up to his responsibilities and withdraw from this election," he said. "This scandal is polluting the campaign."