A sheet storage apparatus for storing a sheet in a storage bag having an opening for inserting the sheet comprising: a supporter configured to support a portion of the storage bag in the vicinity of the opening of the storage bag; an entering unit configured to enter into the storage bag supported by the supporter; a driving unit configured to drive the entering unit; and a controller configured to control 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, wherein the entering unit is further configured to move between a standby position which is a position above the storage bag supported by the supporter and a lower end position where the entering unit enters into the storage bag, the sheet storage apparatus further comprises: a first detector configured to detect whether the entering unit is located at the lower end position; the sheet storage apparatus being characterised in comprising a stage configured to move a bottom of the storage bag supported by the supporter in a substantially vertical direction and on which at least a part of the storage bag is placed, wherein when the entering unit enters into the storage bag and comes in contact with the sheet stored in the storage bag, the controller is configured to control the stage to lower the bottom of the storage bag when it is not detected by the first detector that the entering unit is located at the lower end position, the sheet storage apparatus further comprising, a stopping mechanism configured to stop the entering unit from moving toward the lower end position, when the entering unit comes in contact with the sheet stored in the storage bag before the entering unit reaches the lower end position. a supporter configured to support a portion of the storage bag in the vicinity of the opening of the storage bag; an entering unit configured to enter into the storage bag supported by the supporter; a driving unit configured to drive the entering unit; and a controller configured to control 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, wherein the entering unit is further configured to move between a standby position which is a position above the storage bag supported by the supporter and a lower end position where the entering unit enters into the storage bag, the sheet storage apparatus further comprises: a first detector configured to detect whether the entering unit is located at the lower end position; the sheet storage apparatus being characterised in comprising a stage configured to move a bottom of the storage bag supported by the supporter in a substantially vertical direction and on which at least a part of the storage bag is placed, wherein when the entering unit enters into the storage bag and comes in contact with the sheet stored in the storage bag, the controller is configured to control the stage to lower the bottom of the storage bag when it is not detected by the first detector that the entering unit is located at the lower end position, the sheet storage apparatus further comprising, a stopping mechanism configured to stop the entering unit from moving toward the lower end position, when the entering unit comes in contact with the sheet stored in the storage bag before the entering unit reaches the lower end position. a first detector configured to detect whether the entering unit is located at the lower end position; the sheet storage apparatus being characterised in comprising a stage configured to move a bottom of the storage bag supported by the supporter in a substantially vertical direction and on which at least a part of the storage bag is placed, wherein when the entering unit enters into the storage bag and comes in contact with the sheet stored in the storage bag, the controller is configured to control the stage to lower the bottom of the storage bag when it is not detected by the first detector that the entering unit is located at the lower end position, the sheet storage apparatus further comprising, a stopping mechanism configured to stop the entering unit from moving toward the lower end position, when the entering unit comes in contact with the sheet stored in the storage bag before the entering unit reaches the lower end position.