To obtain a new compound useful for production of pyridazin-3-one derivative having excellent herbicidal activities against weeds such as Ipomoea hederacea var. integriuscula and Abutilon theophrasti.

This new compound is a carboxylic acid derivative of formula I [R2 and R3 are each H or a 1-3C alkyl; Q is a (substituted) phenyl] or a salt thereof. The compound of formula I is obtained by reacting (A) a hydrazone compound of formula II such as 3,3,3-trifluoro-2-oxopropanal 1-(4-chloro-2- fluoro-5-hydroxyphenylhydrazone) with (B) a malonic acid derivative of formula III such as methylmalonic acid in the presence of (C) a secondary amine such as piperidine and (D) pyridine or quinoline in a proportion of 1-10 mol component B and 1-2 mol component C based on 1 mol component A at 60-100°C for a moment to 24 hr.