Can you tell me if automatic sphygmomanometer measure blood pressure as less than it actually is?

Depends on your cuff: Some are very accurate - others not so much. Check yours several times against your doctor's. Take the measurements as simultaneously as possible. Take them 3X and average them for each device because BP is dynamic and always changing. No 2 devices will be perfectly the same but if you're within 10 points, that's pretty good.