Being blank and pretty newbie in Linux, I absolutely had no idea on how to recover from such event. The volume still intact, but might not be if another disk fails.
I search google and found this link. The Software-RAID HOWTO: Reconstruction
All I need to do is just do the command :
raidhotadd /dev/mdX /dev/sdX
It successfully reconstructed the volume yesterday.
This morning, it failed again. I'm trying to reissue the same command again. If it fails again, maybe it's about time to change the harddisk.
(Well, anyway it's an old pentium III, with some used and old SCSI hard disk. 5 SCSI at only 17Gig each.)