An acylphosphine oxide compound represented by Formula 1-1 or Formula 2-1 which can be used as a photopolymerization initiator. In Formula 1-1 and Formula 2-1, A represents an m-valent group, L's each independently represent a single bond or a divalent linking group, R1's each independently represent an alkyl group having 1 to 8 carbon atoms, an aryl group having 6 to 20 carbon atoms, or an alkoxy group having 1 to 4 carbon atoms, R2 and R3 each independently represent an alkyl group, an aryl group, or an alkoxy group, m represents an integer of 3 or more, n1's each independently represent an integer of 0 to 4, and n2's each independently represent an integer of 0 to 5.