Transcranial magnetic stimulation (TMS) is applied as an important method to investigate human cognitive process. In this study, we used TMS to investigate temporal aspect of the right posterior parietal cortex involved in visual search. Subjects were required to respond as quickly and as accurately as possible by pressing a mouse button to indicate the presence or absence of the target, and the reaction times were measured. Subjects received four experiments which the TMS stimulus onset asynchronies (SOA) were set as 100, 150, 200 and 250ms after visual stimulus presentation. When we examined the effects of TMS on pop-out visual search, there was a significant elevation in target-present reaction time when the TMS pluses were applied 150ms after visual stimulus presentation. However for the other SOA cases, there was no significant difference between no-TMS and TMS conditions. Therefore, we considered that the right PPC was involved in the pop-out visual search at about 150ms after visual stimulus presentation.