The present paper analyzes relation between implementation and effectiveness of PBL (Project Based Learning) . PBL has many variation on both process and team configuration, according to what to be learned. But essentially PBL gives good solution clearing the gap between 'can' and 'know' for novices. Novices easily make failures and these are important because 'every failure is a stepping stone to success' . To clear the gap using PBL and to get know-how applicable to an actual development, it is important recognizing and fixing failure in the process, and retrospecting them in a postmortem process. A PBL instructor should not prevent learners to make slight failures, and should guide them to make a small feedback loop for the failure in the process. The retrospection process is also required for the learners to organize their experiences into abstract know-how.