Ashikaga Red Cross Hospital in Tochigi Prefecture said Friday that doctors accidentally left a metal nut inside the heart of a patient who underwent surgery last month, but denied any linkage to the man's death soon after the operation.

"A nut was left by our mistake, but there is no causal linkage to the death," a hospital official claimed.

The patient, whose name is being withheld, underwent the operation to attach an artificial valve to the heart on Oct. 5, the hospital said.

However, doctors accidentally dropped a 1.5-cm nut, part of a medical device for the operation, in the left atrium of his heart.

The hospital found the nut right after the surgery when it took X-rays, and conducted another operation to remove it. The man died Oct. 8.

Police are investigating.