To eliminate requirement for making database of thickness of individual book and to certainly perform vacant space management of a container even when the thickness of the book is varied with time.

The container vacant space management method has an input means 301, 302 for inputting lead-in information or lead-out information; a reading means 301 for reading a book code; a thickness measurement means 301 for measuring the thickness of the book: T; a container conveying means 200 for leading in and out the container relative to a library part; a container lead-in/-out control means 400 for performing control for specifying the container corresponding to the storing information or the lead-out information and leading in or leading out the corresponding container to the lead-in/-out part by the container conveying means; and an operation or renewal means 400 for calculating the vacant space: Ln by operation: Ln=Ln-1+T in the case of leading out and by operation: Ln=Ln-1-T in the case of leading in when the vacant space before leading out the book or before leading in the book in the container is made to Ln-1 and the vacant space after leading out the book or after leading in the book is made to Ln, and data-renewing the free space in the container from Ln-1 to Ln.