The state of emergency, agreed by parliament the day after the attacks and already prolonged once, could be renewed again after it 26 February limit.
"Obviously, we can't rule out that possibility, depending on the level of danger, and we have to act with a great deal of responsibility," Valls told Europe 1 radio.
Human Rights League president Henri Leclerc expressed concern after Valls's statement, saying he feared a "permanent state of emergency".
Fifty-eight well-known French figures on Monday published an appeal to defend the right to demonstrate as a ban on marches in Paris came to an end and a new one on protests on the Champs Elysées and near the Cop21 conference centre was announced.
But only nine were kept in custody on Monday, the others having been arrested for taking part in an unauthorised demonstration rather than involvement in the violence.
And 26 of the more than 300 people placed under house arrest since the Paris attacks have been far-left activists, suspected of planning to disrupt the climate conference rather than terror plots.
There has been a "restriction of freedoms" but it was to "protect our freedoms", Valls said on Tuesday.
A left-wing magistrates' union, the SM, on Tuesday slammed the "exorbitant powers" granted by the state of emergency in a resolution unanimously backed by delegates to its annual conference.
There have been more than 2,000 house searches since the state of emergency was declared, Valls said.
More than 210 people have been taken into custody, 320 weapons, 31 of them weapons of war, have been seized and 250 legal investigations opened.
In the course of the anti-terror raids, police have made 161 drug seizures.
Valls also appealed to tourists to return to Paris.
"Consume," he begged them. "It's the holidays, spend, live, start going to shows, the cinema, the theatre again," adding that their safety was assured.
Cafés and restaurants have seen trade drop 40 per cent since the attacks, according to the trade organisation UIMH and France could lose 0.1 per cent of GDP, ie two billion euros.
US President Barack Obama set the example on Sunday evening, dining with France's François Hollande in a swish restaurant in central Paris.
The menu included fish caught off Brittany with rosemary, potatoes and a shellfish jus, a cheeseboard and a sweet tart brushed with cocoa, washed down with wines from Burgundy, Bordeaux and Banyuls in the south-west.