The present invention is directed to (R)-(-)-4,5-dihydro-5-methyl-6-�4-�(2-propyl-3-oxo-1-cyclohexenyl)amino!phenyl!-3(2H)-pyridazinone, the crystalline form thereof and a method for manufacturing the said compound and the said crystalline form.The compound of the present invention has excellent inhibitory effect on platelet aggregation while giving less side effect, and therefore, it is useful to utilize the compound as an antithrombotic drug. In addition, the compound of the present invention has bronchodialatic effect as well, and it is therefore useful to utilize the compound for chemotherapy of chronic obstructive lung disease, such as asthma and bronchitis.Moreover, the compound of the present invention is useful for chemotherapy of the diseases relating to the concentration of cAMP in cells, such as hypertension, ulcer, diabetes mellitus and cancer.Again, crystalline form of the said compound is the most suitable form in order to supply stable and homogeneous bulk thereof.