Do you know a “400-character manuscript paper”?

Did you guys, who have never lived in Japan, wonder why “Hisho no tori(丕緒の鳥)” was only 50 pages in the magazine, which was said 90 pages long?
In Japanese publishing custom, the number of sheets of the manuscript is converted into the number of “400-character manuscript paper (400字詰め原稿用紙)”. Because in Japanese style writing people does not write with a space between words.
A “400-character manuscript paper (400字詰め原稿用紙)” is the figure right. Nowadays, most of Japanese writers use computers, but, some still write by hand.

Note: The figure is the first page of “Hisho no tori(丕緒の鳥)” I made by my pc.