To evaluate the lowering degree of the quality of foods accompanied by a packaging bag with higher precision by detecting the pH value, resistance value, the pressure of the gas produced in a bag, etc of the liquid to be inspected in the bag as physical quantities and objectively and accurately grasping the absolute values of them.

Either one of the pH value, electric resistance value and the pressure of the gas produced in a packaging bag, etc. of the liquid to be inspected in the packaging bag are detected and the detection result data are converted to the amount of the gas produced in the bag to be stored along with time and the stored detection result data are read in required timing by a non-contact type scanner.