PURPOSE: To obtain an accurate, precise, and safe weighing device even for the small amount of liquid medicine in an automatic liquid medicine weighing device for using in a weight processing device.

CONSTITUTION: Liquid medicine 2 entered into a liquid medicine bottle 1 circulates by three pumps 1 and four filters to remove particles. When the liquid medicine is weighed, the liquid medicine in which particles are removed passes through a check valve 17 and is introduced into a cylinder 6 as a weighing bath. When the liquid medicine is introduced into the cylinder 6, a piston 14 is slowly and accurately operated by a piston driving motor 16. A weighing sensor 9 is set in a position of the amount of liquid medicine to be weighed, and when a sensor detecting line comes to a position of the weighing sensor 9, the sensor detects it to automatically stop the piston 14. After the liquid medicine is completed weighing, when the liquid medicine is compounded within a wafer processing bath 12, the position 14 is pushed out and the weighed liquid medicine passes through a check valve 18 as a liquid medicine supplying valve and is introduced into the wafer processing bath 12.