PARIS, JULY 29: In a newspaper interview, he criticised the fact that one of the two attackers had been released with an electronic tag pending trial on terror charges.
He said the release had been decided by independent judges, not his government.
Mr Valls has faced criticism over the attack near Rouen, and the killing of 84 people by another jihadist in Nice.
Adel Kermiche, one of the two teenagers who attacked the church in Saint-Etienne-du-Rouvray on Tuesday, had been arrested after being caught trying to go to Syria last year.
However he was released with a monitoring tag earlier this year, despite prosecutors' call for his continued detention.
In an interview with Le Monde newspaper published on Friday, Mr Valls called the release a "failure" that must be acknowledged.
He added: "This does not mean that my government is to blame, because that decision was taken by independent judges."
Kermiche and the other attacker, Abdel Malik Petitjean, were shot dead by police outside the church, where they had slit the throat of Fr Jacques Hamel, 86, and taken hostages during Mass.
Petitjean had been on a watch list as a potential security threat since June after trying to enter Syria from Turkey.
After the attack, so-called Islamic State released a video showing the two men pledging allegiance to the group. In another video that emerged on Thursday, Petitjean called for more attacks against states involved in the anti-IS coalition.
Three people have been questioned by police in connection with the church attack. Two have been released and the third, said to be a Syrian man living in a refugee centre, remains in custody.
The French government has faced strong criticism from political opponents over perceived security failings since the attack in Nice.
It was carried out by a Tunisian man, Mohamed Lahouaiej-Bouhlel, who drove a lorry into a crowd at the end of the Bastille Day celebrations.
In his interview, Mr Valls called for a "revolution in our security culture" in view of the jihadist threat.
"Each French person must contribute to collective security," he said. "An era of innocence has come to an end."
