An acid concentration continuous measuring apparatus is equipped with the combination of a body having a flow path through which pickling solution contained in a pickling tank is made to continuously pass in a single direction, a density sensor, a temperature sensor, and a conductivity sensor which are installed on the body and substantially continuously measure the pickling solution flowing along the flow path, and a calculating apparatus which calculates the acid concentration of the pickling solution based on the measured results. Feedback control of the hydrochloric acid concentration in a final pickling tank of a plurality of pickling tanks forming a continuous pickling line is performed using the acid concentration continuous measuring apparatus.