A police school instructor in western Japan accidentally stabbed an officer with a survival knife during a role-playing exercise in December and left him hospitalized for days, police officials said Wednesday.

While the instructor at the Okayama Prefecture training college has said he did not intend to injure the officer, he was fined 500,000 yen ($4,600) for professional negligence resulting in injury and admonished by his boss.

The injured officer quit in January and has filed a damages suit over the case, they said.

During the exercise, the instructor played a criminal wielding a Japanese sword. He was persuaded to drop the weapon but then took out the survival knife and stabbed the officer twice in the chest.

Fake knives are supposed to be used in training, but the survival knife was real, according to the Okayama prefectural police.

While the knife's blade was still covered, one of the wounds penetrated the officer's lung, and the officer spent the next several days in hospital, they said.

In March, local prosecutors summarily indicted the instructor, also a police inspector, and the Okayama Summary Court ordered him to pay the fine the following month.

The Okayama police declined to give further details citing an ongoing trial but said it will deal with the matter sincerely.

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