Photoinitiator, polymerizable composition, ink jet recording method, and acylphosphine oxide compound

Provided is a photoinitiator which is an acylphosphine oxide compound represented by formula 1-1 or formula 2-1. Also provided is a polymerizable composition or an ink jet recording method that use the photoinitiator. Also provided is an acylphosphine oxide compound represented by formula 1-1 or formula 2-1 which can be suitably used as the photoinitiator. In formula 1-1 and formula 2-1, A is an m-valent group, each L is independently a single bond or a divalent linking group, each R 1 is independently 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, R 2 and R 3 are each independently an alkyl group, an aryl group, or an alkoxy group, m is an integer of 3 or more, each n1 is independently an integer of 0 to 4, and each n2 is independently an integer of 0 to 5.