A liquid detergent builder which is highly compatible with a surfactant and gives transparent liquid detergents having extremely excellent detergency, a liquid detergent containing the builder, and a liquid detergent containing an acrylic acid-based polymer (salt) having excellent clay dispersibility and good compatibility with a surfactant. The liquid detergent builder comprises an acrylic acid/maleic acid-based copolymer (salt), wherein the acrylic acid/maleic acid-based copolymer (salt) contains maleic acid (salt) in an amount of 5-90 mol % per mole of the sum of acrylic acid (salt) and maleic acid (salt), and a product (MA.times.Mw) of a weight average molecular weight (Mw) of the copolymer (salt) and a proportion (MA (mol %)) of the maleic acid (salt) in the copolymer (salt) is 450,000 or less.