This study was conducted to establish optimum conditions for coagulation of konjac jelly as well as antimicrobial activity by concentration of $Ca(OH)_2$. Hardness, gumminess, and chewiness of konjac jelly increased according to concentration of konjac powder, the key material of konjac jelly. The highest sensory evaluation score was acquired with konjac jelly made with 3% konjac powder. A $Ca(OH)_2$ concentration of 0.4-0.6% as a coagulation agent was optimum for coagulation of konjac jelly. Further, sensory score was highest at a $Ca(OH)_2$ concentration of 0.6%. All populations of bacteria, yeast, and mold in konjac jelly were restrained by $Ca(OH)_2$ in a concentration- dependent manner. Furthermore, all tested microorganisms were strictly restrained at $1.0{ imes}10^{-2}$ N of $Ca(OH)_2$.