Hi Kazuki,
I had issues with CSV once, please refer to the following forum dialogues. If missing values means that they are null, this will help you:
https://forum.mendix.com/link/questions/99753
If you haven't tried yet, you may want to try this module for all flat file or CSV type of file import:
https://marketplace.mendix.com/link/component/429/Erwin-'t-Hoen/Flat-&-delimited-file-import
I hope these work for you.
Best,
Ferhat