Thank you for your help.
As you said, it was a list style, so I was able to hide it.
But there was still a problem.
I checked the parent theme.
Can you see the sample image?
This is a preview of the parent theme, which does not include own customization.
Related posts of normal posts are displayed in the same style as the demo, so there is no problem.
The problem occurs in a category that contains one “Sticky Posts”.
Perhaps sticky post does not reflect css?
So I think that this problem is not for my customization, but what do you think?
I want to delete the blank spaces displayed only in the case of the “Sticky Posts”.
Thanks you.
https://www.u510.net/sample_img/sample-2018-08-10.png