"There are obviously mistakes for which I bear responsibility," Laschet said in a statement on Twitter, as he admitted to not properly citing sources in his 2009 book.
"I would like to apologise," said the head of Merkel's CDU party, adding that he would commission a review of the book to establish whether any other errors had been made.
Author and development expert Karsten Weitzenegger tweeted on Thursday that a German "plagiarism hunter" had informed him that Laschet had lifted entire sentences from his own work on integration, published a year before.
Annalena Baerbock, the Green party's chancellor candidate, was also accused of plagiarising passages of a book in June.
The scandal was one of a series of blunders that left the Greens trailing badly in the polls, having briefly overtaken the conservatives in April.
Baerbock was also criticised for her failure to declare to parliament a bonus she had received from the party, as well as inaccuracies on her CV that have since been corrected.
But more recently, Laschet has been on the back foot after he was caught on camera laughing in the background during a tribute to the victims of Germany's recent deadly floods.
In a survey for the RTL and NTV broadcasters published Friday, the conservative CDU/CSU alliance was down to 28 percent, while the Greens rose one point to 21 percent.
