While spawning and planula release are known to occur during night time in the majority of coral species, a species endemic to high latitude of Japan and Korea, Alveopora japonica, was found to release planulae during daytime. Laboratory experiments were conducted to see how light affects planula release in this species. Under the natural light condition in the laboratory A. japonica released planulae between 07:00 and 12:00 in the morning. When the timing of exposure to light was changed, planulation also shifted and coincided with the time of light exposure irrespective of the actual time of day. When the colonies were exposed to light twice a day, i. e. in the morning and evening, all colonies released planulae only in the morning. Direct observation on planulating polyps showed that polyps elongated before planulation and such polyp elongation occurred only under the light condition. Consequently, it is concluded that light is a proximate factor which governs planula release through polyp elongation in A. japonica.