An optical device includes a first region and an isolating layer which are each provided in a semiconductor substrate. The first region configures a photoelectric converter and includes at least an impurity of a first conductivity type. The isolating layer is configured to inhibit passage of electrons. The isolating layer includes a second region which is below the first region and which includes an impurity of a second conductivity type, a third region which surrounds the first region in plan-view thereof and which includes an impurity of the second conductivity type, and a fourth region which surrounds the second region in plan-view thereof and which is connected to the third region. The fourth region is greater in width than a connecting part of the third region which connects the third region to the fourth region.