Errors

valid_signature
gpg: Signature made Tue 08 Oct 2019 09:00:26 PM JST
gpg:                using RSA key C2E15D4EF1D26573A85F25F03521A36D7CDCA07E
gpg: requesting key 3521A36D7CDCA07E from hkp server keyserver.ubuntu.com
gpg: Can't check signature: No public key
==> BAD/TAMPERED signature detected! <==