To provide an unsealing device for a bag with which the opening of a bag body can be surely and easily opened.

A separating mechanism 30 is driven, the lower part of a film material (f) cut-off part or the opening Fa of a bag body F is sucked and held by a pair of right and left suckers 34, and a part near the material (f) cut-off part or the opening Fa of the body F is pressed with a resistance imparting member 35. The film material (f) is cut and heat sealed roughly in the same length corresponding to a box size, and after separating as one bag body F, the suckers 34 are relatively moved to the opposing directions for displacing and for separating the inside face of the mouth part Fa of the bag F in a face direction. The suckers 34 are relatively moved in a separating direction, and the opening Fa of the body F held by the suckers 34 is opened.