1. A proportional controlling system which controls a physical quantity of an objective by making a manipulating quantity operate on said objective and which has a first means for detecting said physical quantity, a second means for outputting a difference signal reflecting the difference between said physical quantity and a predetermined reference value, and a third means capable of generating said manipulating quantity in proportion to said difference signal, said proportional controlling system comprising: an equilibrium-state detecting means for detecting that said physical quantity has been stabilized at a constant value; a memory for memorizing said constant value; an adding means for adding said constant value to said difference signal outputted from said second means; and a manipulating quantity changing means for making said third means generate a new manipulating quantity in accordance with the output from said adding means, thereby manipulating said objective in porportion to the sum of said constant value and said difference signal, once said physical quantity is stabilized at said constant value.