Hello Keisuke,
like described in the SAP S/4HANA conversion guide (link in SAP Help or via my blog The System Conversion to SAP S/4HANA, on-premise edition 1511 - Technical procedure and semantic adaption tasks):
we recommend the following step sequence:
1. Maintenance Planner
2. Pre-Conversion Checks
3. Custom Code Checks
4. SUM
More details in the conversion guide.
Kind Regards,
Frank