The steam-releasing flat pouch comprises a seal part, a steam-releasing seal part, and a steam discharge part. The seal part seals stacked sheets. The steam-releasing seal part is located in the inner parts of the sheet inside the seal part, and is configured to join parts of the sheet so that they are not continuous with the seal part. The steam discharge part is formed in the steam-releasing seal part and configured not to allow communication with the outside when the steam-releasing seal part is not peeled, and to allow communication with the outside when the steam-releasing seal part is peeled. The steam-releasing seal part includes an anterior peeling part including a part that peels apart first as the pressure increases, and a posterior peeling part including a part that peels apart last. The width of the posterior peeling part is smaller than the width of the anterior peeling part.