The, present invention relates to a novel allylthiopyridazine derivative represented by formula (I) which exhibits a superior effect for prevention and treatment or hepatic diseases induced by toxic substances and for protection of human tissues from radiation: ##STR1## or a pharmaceutically acceptable salt thereof, in which R.sub.1 represents halogen atom, lower alkoxy, dialkylaminoalkoxy, hydroxyalkoxy, phenoxy substituted or unsubstituted with lower alkyl, benzyloxy, or phenyl, andR.sub.2 and R.sub.3 independently of one another represent hydrogen or lower alkyl, or R.sub.2 and R.sub.3 together with carbon atom to which they are attached can form a saturated or unsaturated 6-membered ring,provided that R.sub.2 and R.sub.3 are other than hydrogen when R.sub.1 is chloro; and to a process for preparing thereof and a pharmaceutical composition containing the same as an effective component.