A method for manufacturing a solid state image pickup device including a first active region provided with a first conversion unit, a second active region provided with a second conversion unit, and a third active region adjoining the first and the second active regions with a field region therebetween and being provided with a pixel transistor, the method including the steps of ion-implanting first conductivity type impurity ions to form a semiconductor region serving as a potential barrier against the signal carriers at a predetermined depth in the third active region and ion-implanting second conductivity type impurity ions into the third active region with energy lower than the above-described ion-implantation energy.