A sheet storage apparatus (for example, a banknote storage mechanism) includes an entering unit (for example, a pushing unit) configured to enter into a storage bag (for example, a banknote storage bag) supported by a supporter (for example, holding members) and a controller configured to control the driving unit (for example, a motor) for driving the entering unit. The controller controls the driving unit such that the entering unit enters into the storage bag supported by the supporter and comes in contact with the sheet stored in the storage bag to detect an upper end position of the sheet stored in the storage bag.