A ventilation unit configured to be attached to a housing of a battery pack , the ventilation unit comprising: an internal pressure adjusting component ; wherein the internal pressure adjusting component comprises a support body on which a communicating hole for communicating the inside of the housing with the outside of the housing is formed; a first ventilation body that permits gas to flow from an inside of the housing to an outside of the housing when internal pressure, which is pressure inside the housing , is higher than external pressure, which is pressure outside the housing , by a value not less than a predetermined value, and is capable of returning to block flow of the gas from the inside of the housing to the outside of the housing ; wherein the internal pressure adjusting component comprises a second ventilation body that permits gas to flow between the inside of the housing and the outside of the housing even when a pressure difference between the internal pressure and the external pressure is less than the predetermined value, wherein the first ventilation body includes an annular portion having an annular shape provided to a center and having a through hole, and an inclined portion extending from an outer circumferential part of the annular portion in a direction inclined with respect to a centerline direction, characterized in that the second ventilation body is a ventilation membrane that is attached to the support body to cover the communicating hole so as to permit flow of gases between the inside of the housing and the outside of the housing and to prevent liquids and solids from penetrating into the inside of the housing from the outside of the housing . an internal pressure adjusting component ; wherein the internal pressure adjusting component comprises a support body on which a communicating hole for communicating the inside of the housing with the outside of the housing is formed; a first ventilation body that permits gas to flow from an inside of the housing to an outside of the housing when internal pressure, which is pressure inside the housing , is higher than external pressure, which is pressure outside the housing , by a value not less than a predetermined value, and is capable of returning to block flow of the gas from the inside of the housing to the outside of the housing ; wherein the internal pressure adjusting component comprises a second ventilation body that permits gas to flow between the inside of the housing and the outside of the housing even when a pressure difference between the internal pressure and the external pressure is less than the predetermined value, wherein the first ventilation body includes an annular portion having an annular shape provided to a center and having a through hole, and an inclined portion extending from an outer circumferential part of the annular portion in a direction inclined with respect to a centerline direction, characterized in that the second ventilation body is a ventilation membrane that is attached to the support body to cover the communicating hole so as to permit flow of gases between the inside of the housing and the outside of the housing and to prevent liquids and solids from penetrating into the inside of the housing from the outside of the housing . <EOS>