Phosphorus-containing oligomer and method for producing the same, curable resin composition and cured product of the same, and printed wiring board

A method for producing a phosphorus-containing oligomer, the method comprising mixing a compound (a1) represented by structural formula (a1-1) or (a1-2) below and a compound (a2) represented by structural formula (a2) below with each other at a molar ratio of [compound (a1)/compound (a2)]=0.01/1.0 to 0.99/1.0;

• causing a reaction to proceed at 80° C. to 180° C. in the presence of an acid catalyst;
• then adding the compound (a1) so that the total amount on a molar basis is 1.01 to 3.0 times the amount of the compound (a2) charged;
• and causing a reaction to proceed at 120° C. to 200° C.,
• (in the formula, R2, R3, R4, and R5 each independently represent a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, a phenyl group, or an aralkyl group)
• (in the formula, R1 represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, an alkoxy group having 1 to 4 carbon atoms, or a phenyl group).