A biphenyl ammonium compound represented by formula (I) or a salt thereof, which has a BETA2 adrenergic receptor agonist activity and a muscarin receptor antagonist activity. R1 is H or a C1-8 alkyl. R4 is a C1-10 alkylene. R2 is a single bond, a (substituted) C1-8 alkylene, a C2-4 alkenylene, or an O-(C1-4 alkylene). X is a single bond, O, CONR3, NR3CO, or NR3COCH2O (wherein R3 is H or a C1-8 alkyl). A1 is a single bond, a (substituted) C6-10 arylene, etc. R8 and R9 each is X, CN, a C1-6 alkyl, etc. a and b each is 0-3. W- is an anion. Y is any of formulae (1) to (9). Q is any of formulae (10) to (12) (wherein R6 and R7 each is a (substituted) C1-6 alkyl or a (substituted) C8-10 phenoxyalkyl； n is 0-2； m is 0-3； and R5 is a (substituted) C1-6 alkyl (two of R5 to R8 may form a ring structure)).
