An oven including: a cavity forming an inner cooking space; a door pivotably installed at a front opening of the cavity for covering the cavity; and a heater provided within the cavity for generating heat, wherein the door comprises: a plurality of door panels, the plurality of door panels being configured to be stacked with predetermined spaces therebetween to define door cooling passages; and a pair of side frames for supporting side edges of the plurality of door panels, wherein the plurality of door panels include: an inner door panel placed on the pair of side frames, the inner door panel being exposed to an inner space of the cavity; an outer door panel on which the pair of side frames are mounted, the outer door panel being exposed to air outside the cavity; and one or more middle door panels (182,183) disposed between the inner door panel and the outer door panel, wherein the pair of side frames is disposed at the rear of the outer door panel by a predetermined distance such that a side intake for air is formed in that gap, wherein outside air flows, in use, through said side intake to cool the door, wherein each of the side frames includes a supporting portion, the supporting portion comprising: an outer supporting portion; an inner supporting portion spaced a predetermined distance apart from the outer supporting portion towards a center of the door; and characterized in that: the inner door panel is mounted on the inner supporting portion and on the outer supporting portion, an air receiving hole is formed as a sealed space between the inner supporting portion, the outer supporting portion and the inner door panel, and the air receiving hole acts as a thermal barrier layer that blocks transfer of heat from the cavity. a cavity forming an inner cooking space; a door pivotably installed at a front opening of the cavity for covering the cavity; and a heater provided within the cavity for generating heat, wherein the door comprises: a plurality of door panels, the plurality of door panels being configured to be stacked with predetermined spaces therebetween to define door cooling passages; and a pair of side frames for supporting side edges of the plurality of door panels, wherein the plurality of door panels include: an inner door panel placed on the pair of side frames, the inner door panel being exposed to an inner space of the cavity; an outer door panel on which the pair of side frames are mounted, the outer door panel being exposed to air outside the cavity; and one or more middle door panels (182,183) disposed between the inner door panel and the outer door panel, wherein the pair of side frames is disposed at the rear of the outer door panel by a predetermined distance such that a side intake for air is formed in that gap, wherein outside air flows, in use, through said side intake to cool the door, wherein each of the side frames includes a supporting portion, the supporting portion comprising: a plurality of door panels, the plurality of door panels being configured to be stacked with predetermined spaces therebetween to define door cooling passages; and a pair of side frames for supporting side edges of the plurality of door panels, wherein the plurality of door panels include: an inner door panel placed on the pair of side frames, the inner door panel being exposed to an inner space of the cavity; an outer door panel on which the pair of side frames are mounted, the outer door panel being exposed to air outside the cavity; and one or more middle door panels (182,183) disposed between the inner door panel and the outer door panel, an inner door panel placed on the pair of side frames, the inner door panel being exposed to an inner space of the cavity; an outer door panel on which the pair of side frames are mounted, the outer door panel being exposed to air outside the cavity; and one or more middle door panels (182,183) disposed between the inner door panel and the outer door panel, wherein the pair of side frames is disposed at the rear of the outer door panel by a predetermined distance such that a side intake for air is formed in that gap, wherein outside air flows, in use, through said side intake to cool the door, wherein each of the side frames includes a supporting portion, the supporting portion comprising: an outer supporting portion; an inner supporting portion spaced a predetermined distance apart from the outer supporting portion towards a center of the door; and characterized in that: the inner door panel is mounted on the inner supporting portion and on the outer supporting portion, an air receiving hole is formed as a sealed space between the inner supporting portion, the outer supporting portion and the inner door panel, and the air receiving hole acts as a thermal barrier layer that blocks transfer of heat from the cavity. characterized in that: the inner door panel is mounted on the inner supporting portion and on the outer supporting portion, an air receiving hole is formed as a sealed space between the inner supporting portion, the outer supporting portion and the inner door panel, and the air receiving hole acts as a thermal barrier layer that blocks transfer of heat from the cavity. the inner door panel is mounted on the inner supporting portion and on the outer supporting portion, an air receiving hole is formed as a sealed space between the inner supporting portion, the outer supporting portion and the inner door panel, and the air receiving hole acts as a thermal barrier layer that blocks transfer of heat from the cavity.