To provide a rust preventive for an ALC (autoclaved lightweight concrete) reinforcing bar capable of obtaining a rust preventive film having excellent adhesion with a reinforcing bar and rust preventive performance and further having excellent water resistance, and suitable for the production of an ALC panel free from coloring in the vicinity of a hole at the side face upon the drawing-out of a fixing rod used upon its arrangement at a mold, and also having excellent appearance quality.

A rust preventive for an ALC reinforcing bar is includes slurry obtained by mixing slaked lime for pH control and water for viscosity control into SBR (Styrene Butadiene Rubber) water-based emulsion in which the bonding amount of styrene is 65 to 75 wt.%, the water- based emulsion of asphalt and the carbonate powder of alkaline-earths. As the carbonate powder, calcium carbonate powder having the average grain size of 8 to 30 m and whose specific surface area by an air permeability method is 2,000 to 50,000 cm2/g is used, and further, the total of the water in the water- based emulsion and the water for viscosity control is controlled to 15 to 25 wt.% to the whole slurry.