Sorry, I don't speak Japanese but I'll respond in English. IPv6 support on MX is very limited at the moment:
"Cisco Meraki security appliances can pass IPv6 traffic in pass-through mode, but no traffic analysis or manipulation is possible when using IPv6. At this time, IPv6 traffic cannot be routed or passed along a VPN tunnel."
Source:
https://documentation.meraki.com/zGeneral_Administration/Other_Topics/IPv6_Device_Compatibility
To keep up-to-date about IPv6 support, subscribe to this topic:
https://community.meraki.com/t5/Full-Stack-Network-Wide/IPv6-Meraki/m-p/53677
