To obtain a new heterotricyclic herbicide comprising a specific heterotricyclic compound, its N-oxide or an agrichemically suitable salt thereof and useful as an inexpensive, low-toxic and environmentally safe herbicide for controlling undesirable trees and plants.

This new heterotricyclic compound is represented by formula I {Q is a group of formula II [Z is CH2CH2CH2, OCH2CH2, OCH=CH, OCH2O, CH=NCH2, CH2CH2, etc.; R4 is a (substituted) phenylthio or a (substituted) phenylsulfonyl], etc.; X is O, S(O)n [(n) is 0-2], N, CH, CH2, etc., which is replaced with the or two 1-2C alkyl groups; A is O or S(O)n; B is S(O)n; R1 is a 1-6C (halo)alkyl, a 1-6C (halo)alkoxy, etc.; R2 is a 1-3C alkyl; R3 is a 1-2C alkyl; (m) and (p) are each 0-2; (q) is 1 or 2; (r) is 0-3; (s) is 0-4}, its N-oxide or a salt and is useful as a herbicide, etc. The compound is obtained by reacting an acid chloride of formula III with a dione of formula IV in the presence of a base such as triethylamine.