Abstract: A pyrrolopyridazine compound having the formula (I) or a pharmaceutically acceptable salt thereof: wherein, R1 is a C2-C6 alkenyl group, a halogeno C2-C6 alkenyl group, a C3-C7 cycloalkyl group which may be optionally substituted or a C3-C7 cycloalkyl- C1-C6 alkyl group which may be optionally substituted. R2 is a C1-C6 alkyl group. R3 is a hydroxymethyl group, a C2-C6 aliphatic acyloxymethyl group, a C6-C10 arylcarbonyloxymethyl group which may be optionally substituted, a C1-C6 alkoxycarbonyloxymethyl group, a formyl group, a carboxyl group, a C1-C6 alkoxycarbonyl group or a C6-C10 aryloxycarbonyl group which may be optionally substituted. R4 is a C6-C10 aryl group which may be optionally substituted. A is an imino group, an oxygen atom or a sulfur atom. These compounds exhibit excellent gastric acid secretory inhibition activity and gastric mucous membrane protection activity etc. They are useful for prevention or treatment of ulcerative diseases and for Helicobacter pylori infections.