that節のthatと関係代名詞のthat

that節
He said that ………文………
He argued that ………文………

関係代名詞のthat
I like the train that runs fast. 主格の関係代名詞
I like the book that I bought last month.目的格の関係代名詞

He said thatと I like the book that I bought last month.のthatは省略ができる。