An olfactory display which emits odors just in front of the nose can control their strength temporally and spatially. It can enhance their benefit by adjusting their strength according to a user's condition. We examined their effects on user's tasks when we control their strength according to user's concentration ratio, which we estimated from his/her heart rate fluctuation (RRV). Miss rates of user's tasks decreased 30% on average when the display emits odors when user's concentration ratio is low compared to when it doesn't. Moreover we found it is more effective to control them than to emit it constantly or off and on.