The head of the Osaka Prefectural Police headquarters says it is extremely regrettable that a gun was taken from the police officer who was stabbed on Sunday morning.
The gun was loaded with five bullets.
Takahisa Ishida told reporters that Osaka police have launched a massive manhunt and are making every effort to arrest the culprit as soon as possible.
He suggested that the suspect should not remain at large with the gun ahead of the Group of 20 summit on June 28 and 29.
Osaka will host the summit, which will be attended by the leaders of more than 30 countries and international organizations, including the United States, China and Russia.
Tight security is already in place, with forces from Tokyo and other prefectures involved.
Police plan to further increase security to avert terrorism and protect dignitaries.