To provide a thickness detecting device for a filling/packing bag capable of accurately detecting a boundary line caused by a thickness between the bag-like section and lateral sealed portion of a contiguous belt-shaped filling/packing bag and performing a stable follow-up motion, without being affected by the quantity of a content such as a liquid and a powder to be housed.

Since a thickness detecting mechanism 5 capable of detecting a difference in thickness between the bag-like section FP and the lateral sealed section FP formed in the contiguous filling/packing P is provided with sensors 13 which follow up an oscillation in the lateral direction of a packing bag to a direction where the packing bag is transferred and are opposed to each other from both external sides of the films of the packing bag and can press and contact the films and a sensor 16 which detects the displacement of the sensors by the difference in thickness between the bag-like section FP and the lateral sealed section FS to constitute the thickness detector of the filling/packing bag, although a packing form by the kind of the content, particularly the thickness of the packing bag is changed, a follow-up characteristic is good to the rolling of the packing bag. And the detecting device can be operated in a stable condition without being affected by the variations of thickness of the bag-like section in the packing bag.