Afforded are a polishing agent, and a compound semiconductor manufacturing method and semiconductor device manufacturing method utilizing the agent, whereby the surface quality of compound semiconductor substrates can be favorably maintained, and high polishing rates can be sustained as well. The polishing agent is a polishing agent for GaαIn(1-α)AsβP(1-β) (0≦α≦1; 0≦β≦1) compound semiconductors, and includes an alkali metal carbonate, an alkali metal organic salt, a chlorine-based oxidizer, and an alkali metal phosphate, wherein the sum of the concentrations of the alkali metal carbonate and the alkali metal organic salt is between 0.01 mol/L and 0.02 mol/L, inclusive. The compound semiconductor manufacturing method comprises a step of preparing a GaαIn(1-α)AsβP(1-β) (0≦α≦1; 0≦β≦1) compound semiconductor, and a step of polishing the face of the compound semiconductor utilizing an aforedescribed polishing agent.