An imidazopyridazine compound represented by the following formula: (wherein R 1 represents a halo (C 1 -C 6) alkyl group; a halo (C 1 -C 6) alkoxy group; a (C 3 -C 6) cycloalkyl group; a halo (C 1 -C 6) alkylthio group; a halo (C 1 -C 6) alkylsulfinyl group; or a halo (C 1 -C 6) alkylsulfonyl group, R 2 represents a halogen atom; a (C 1 -C 6) alkoxy group; a (C 1 -C 6) alkylthio group; a (C 2 -C 6) alkenyl group; a (C 3 -C 6) cycloalkyl (C 2 -C 6) alkynyl group; an N(R 4)CON(R 4)R 5 group wherein R 4 and R 5 may be the same or different and each represent a hydrogen atom, a (C 1 -C 6) alkyl group, a halo (C 1 -C 6) alkyl group, a (C 1 -C 6) alkoxycarbonyl group, a (C 1 -C 6) alkylcarbonyl group, or a (C 3 -C 6) cycloalkylcarbonyl group; an N(R 4)R 5 group wherein R 4 and R 5 are the same as above; an aryl group; an aryl group having, on the ring, 1 to 3 substituting groups which may be the same or different and are selected from (a) a halogen atom, (b) a cyano group, (c) a nitro group, (d) a (C 1 -C 8) alkyl group, (e) a halo (C 1 -C 8) alkyl group, (f) a (C 1 -C 8) alkoxy group, (g) a halo (C 1 -C 8) alkoxy group, (h) a (C 1 -C 8) alkylthio group, (i) a halo (C 1 -C 8) alkylthio group, (j) a (C 1 -C 8) alkylsulfinyl group, (k) a halo (C 1 -C 8) alkylsulfinyl group, (l) a (C 1 -C 8) alkylsulfonyl group, and (m) a halo (C 1 -C 8) alkylsulfonyl group; a heterocyclic group; or a heterocyclic group having, on the ring, 1 to 3 substituting groups which may be the same or different and are selected from (a) a halogen atom, (b) a cyano group, (c) a nitro group, (d) a (C 1 -C 8) alkyl group, (e) a halo (C 1 -C 8) alkyl group, (f) a (C 1 -C 8) alkoxy group, (g) a halo (C 1 -C 8) alkoxy group, (h) a (C 1 -C 8) alkylthio group, (i) a halo (C 1 -C 8) alkylthio group, (j) a (C 1 -C 8) alkylsulfinyl group, (k) a halo (C 1 -C 8) alkylsulfinyl group, (l) a (C 1 -C 8) alkylsulfonyl group, and (m) a halo (C 1 -C 8) alkylsulfonyl group, R 3 represents a (C 1 -C 6) alkylthio (C 1 -C 6) alkyl group; a (C 1 -C 6) alkylsulfinyl (C 1 -C 6) alkyl group; or a (C 1 -C 6) alkylsulfonyl (C 1 -C 6) alkyl group, and Et represents an ethyl group), or a salt thereof. R 1 represents a halo (C 1 -C 6) alkyl group; a halo (C 1 -C 6) alkoxy group; a (C 3 -C 6) cycloalkyl group; a halo (C 1 -C 6) alkylthio group; a halo (C 1 -C 6) alkylsulfinyl group; or a halo (C 1 -C 6) alkylsulfonyl group, R 2 represents a halogen atom; a (C 1 -C 6) alkoxy group; a (C 1 -C 6) alkylthio group; a (C 2 -C 6) alkenyl group; a (C 3 -C 6) cycloalkyl (C 2 -C 6) alkynyl group; an N(R 4)CON(R 4)R 5 group wherein R 4 and R 5 may be the same or different and each represent a hydrogen atom, a (C 1 -C 6) alkyl group, a halo (C 1 -C 6) alkyl group, a (C 1 -C 6) alkoxycarbonyl group, a (C 1 -C 6) alkylcarbonyl group, or a (C 3 -C 6) cycloalkylcarbonyl group; an N(R 4)R 5 group wherein R 4 and R 5 are the same as above; an aryl group; an aryl group having, on the ring, 1 to 3 substituting groups which may be the same or different and are selected from (a) a halogen atom, (b) a cyano group, (c) a nitro group, (d) a (C 1 -C 8) alkyl group, (e) a halo (C 1 -C 8) alkyl group, (f) a (C 1 -C 8) alkoxy group, (g) a halo (C 1 -C 8) alkoxy group, (h) a (C 1 -C 8) alkylthio group, (i) a halo (C 1 -C 8) alkylthio group, (j) a (C 1 -C 8) alkylsulfinyl group, (k) a halo (C 1 -C 8) alkylsulfinyl group, (l) a (C 1 -C 8) alkylsulfonyl group, and (m) a halo (C 1 -C 8) alkylsulfonyl group; a heterocyclic group; or a heterocyclic group having, on the ring, 1 to 3 substituting groups which may be the same or different and are selected from (a) a halogen atom, (b) a cyano group, (c) a nitro group, (d) a (C 1 -C 8) alkyl group, (e) a halo (C 1 -C 8) alkyl group, (f) a (C 1 -C 8) alkoxy group, (g) a halo (C 1 -C 8) alkoxy group, (h) a (C 1 -C 8) alkylthio group, (i) a halo (C 1 -C 8) alkylthio group, (j) a (C 1 -C 8) alkylsulfinyl group, (k) a halo (C 1 -C 8) alkylsulfinyl group, (l) a (C 1 -C 8) alkylsulfonyl group, and (m) a halo (C 1 -C 8) alkylsulfonyl group, R 3 represents a (C 1 -C 6) alkylthio (C 1 -C 6) alkyl group; a (C 1 -C 6) alkylsulfinyl (C 1 -C 6) alkyl group; or a (C 1 -C 6) alkylsulfonyl (C 1 -C 6) alkyl group, and Et represents an ethyl group), or