Tuesday's attacker had "never showed any sign of radicalisation," he told RTL radio, but stressed that the investigation was at an early stage.
Castaner said it "is very difficult to anticipate" attacks when they were staged by people who were not on the police radar.
But he said the police response was "immediate and effective."
The 22-year-old officer sustained minor neck injuries in the assault, which comes with France on high alert after jihadists killed seven people in London on Saturday.
Documents found on the attacker identified him as a 40-year-old Algerian who was a doctoral student in information science at a university in the east of France.
The suspect later claimed to be a "soldier of the caliphate" of the Islamic State group, according to a source close to the investigation.
The policeman's colleague opened fire on the man, hitting him in the chest in panicked scenes around the Gothic cathedral that is one of France's most visited tourist attractions.
France is still under the state of emergency imposed after the November 2015 attacks in Paris, when ISIS jihadists killed 130 people in a night of carnage at venues across the city.