A centrifugal air blower comprising: a fan including a disk-shaped main plate and a plurality of blades disposed on a peripheral portion of the main plate; and a scroll casing including: a sidewall covering the fan from an axial direction of a rotation axis on which the fan rotates, the side wall having a suction opening for sucking air; a discharge opening for discharging an airflow generated by the fan; a tongue portion for guiding the airflow to the discharge opening; a peripheral wall surrounding the fan from a radial direction of the rotation axis, the peripheral wall extending in a direction of rotation of the fan from a first end portion of the discharge opening located on a side of the tongue portion to a second end portion of the discharge opening located away from the tongue portion; and a bell mouth formed along the suction opening of the sidewall, wherein the bell mouth includes an upstream end and a downstream end, the upstream end being an end portion on an upstream side in a direction of flow of the air passing through the suction opening, the downstream end being an end portion on a downstream side in the direction of flow of the air, at all locations between the first end portion and the second end portion, a distance in the radial direction of the rotation axis between the upstream end and the downstream end of the bell mouth becomes longer as an angle (θ) relative to the first end portion of the discharge opening in the direction of rotation of the fan increases, a distance in the axial direction of the rotation axis between the upstream end and the downstream end is longer than a distance in a direction perpendicular to the rotation axis between the upstream end and the downstream end at all circumferential locations of the bell mouth, and a curvature of the bell mouth from the upstream end to the downstream end is smaller than a curvature of a bell mouth providing an arc-shaped cross-section where an axial distance between an upstream end and a downstream end and a radial distance between the upstream end and the downstream end are equal to each other, characterized in that a position of the upstream end of the bell mouth in the axial direction of the rotation axis remains constant. a fan including a disk-shaped main plate and a plurality of blades disposed on a peripheral portion of the main plate; and a scroll casing including: a sidewall covering the fan from an axial direction of a rotation axis on which the fan rotates, the side wall having a suction opening for sucking air; a discharge opening for discharging an airflow generated by the fan; a tongue portion for guiding the airflow to the discharge opening; a peripheral wall surrounding the fan from a radial direction of the rotation axis, the peripheral wall extending in a direction of rotation of the fan from a first end portion of the discharge opening located on a side of the tongue portion to a second end portion of the discharge opening located away from the tongue portion; and a bell mouth formed along the suction opening of the sidewall, wherein a sidewall covering the fan from an axial direction of a rotation axis on which the fan rotates, the side wall having a suction opening for sucking air; a discharge opening for discharging an airflow generated by the fan; a tongue portion for guiding the airflow to the discharge opening; a peripheral wall surrounding the fan from a radial direction of the rotation axis, the peripheral wall extending in a direction of rotation of the fan from a first end portion of the discharge opening located on a side of the tongue portion to a second end portion of the discharge opening located away from the tongue portion; and a bell mouth formed along the suction opening of the sidewall, wherein the bell mouth includes an upstream end and a downstream end, the upstream end being an end portion on an upstream side in a direction of flow of the air passing through the suction opening, the downstream end being an end portion on a downstream side in the direction of flow of the air, at all locations between the first end portion and the second end portion, a distance in the radial direction of the rotation axis between the upstream end and the downstream end of the bell mouth becomes longer as an angle (θ) relative to the first end portion of the discharge opening in the direction of rotation of the fan increases, a distance in the axial direction of the rotation axis between the upstream end and the downstream end is longer than a distance in a direction perpendicular to the rotation axis between the upstream end and the downstream end at all circumferential locations of the bell mouth, and a curvature of the bell mouth from the upstream end to the downstream end is smaller than a curvature of a bell mouth providing an arc-shaped cross-section where an axial distance between an upstream end and a downstream end and a radial distance between the upstream end and the downstream end are equal to each other, characterized in that a position of the upstream end of the bell mouth in the axial direction of the rotation axis remains constant.