hello koichi.tanoiri,
please see-also Topic 301708, it means the signing key is different from your kernel building.
here’re couple of approaches to resovle this.
(1) you may configure CONFIG_MODULE_SIG as disabled to get rid of the message during modprobe/insmod.
or…
(2) you may try to specify your own sig_key using CONFIG_MODULE_SIG_KEY in kernel config, to rebuild the kernel and kernel modules, install the kernel image, dtbs and kernel modules to target. after that, please use your own sig_key to sign the *.ko built later.
please see-also developer guide for reference, [Kernel Module Signing].