The present invention provides a low alloy steel and a weld joint thereof excellent in hydrochloric acid corrosion resistance and sulfuric acid corrosion resistance, said low alloy steel containing, in mass, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, Cu: 0.1 to 1%, Mo: 0.001 to 1%, Sb: 0.01 to 0.2%, P: 0.05% or less, and S: 0.05% or less, with the balance consisting of Fe and unavoidable impurities; and the acid corrosion resistance index AI of said low alloy steel being zero or positive. Here, said AI is given by the following expression, AI/10,000=0.0005+0.045×Sb %−C %×Mo %, where % means mass %.