Hi,
I found the reason of the message.
My customer uses "F-secure" antivirus software.
There are several pages that indicate F-secure causes this message such as [1].
I told my customer to stop F-secure and report to the vendor.
Anyway, I think this message is not helpful to administrators and should be lower level such as "DEBUG1".
[1] - https://www.postgresql.org/message-id/flat/feb75b80-aa4e-0b45-fbe7-0c2335904fcc%40evolu-s.it
Regards,
Ryohei Takahashi