A preventive, therapeutic or improving agent for diseases against which activation of the
thrombopoietin receptor is effective or a
platelet increasing agent, which contains a
thrombopoietin receptor activator represented by the formula (1): [wherein each of R1 and R3 is independently a
hydrogen atom, SO3H, a C1-6
alkyl group, a C1-6 alkylcarbonyl group or a C6-18 arylcarbonyl group (the C1-6
alkyl group, the C1-6 alkylcarbonyl group and the C6-18 arylcarbonyl group may be optionally substituted with a
halogen atom, a hydroxyl group, a C2-6 alkenyl group, a C1-6
alkoxy group, a C1-6 alkoxycarbonyl group, a C6-18
aryl group, a 2-pyridyl group, a 3-pyridyl group, a 4-pyridyl group, a 2-furanyl group, a 3-furanyl group, a 2-thienyl group, a 3-thienyl group or NR9R10) , and each of R2, R4 and Ra is independently a
hydrogen atom, a hydroxyl group or a C1-6
alkoxy group].