Thank you very much. It works. 
I knew line-number-display-limit but didn't know line-number-display-limit-width.

> From: drew.adams@oracle.com
> To: schwab@linux-m68k.org; netjunegg@outlook.com
> Subject: RE: with text file including very long lines,	line number field of mode line becomes "??"
> Date: Fri, 30 Nov 2012 08:47:30 -0800
> CC: emacs-devel@gnu.org
> 
> > See line-number-display-limit and line-number-display-limit-width.
> 
> Maybe the popup menu over the line number could have an item that takes you to
> the doc for those vars when `??' is displayed: (emacs) `Optional Mode Line'.
> 
> IOW, `??' says only "No idea what the line number is".  It could help to also
> point users to info on how to control/fix this.
> 
> And/or a popup item could just increase the limit to make it big enough to show
> the number of the last line number for the current buffer.
> 
>