We investigated the inter-brain activity coherence recorded by an EEG between two participants when playing at Kokkuri-san, a Japanese Ouija board. The Ouija aboard, known as a spirit key board or talking board, is a flat board, marked with the letters of the alphabet, numbers, or words (“yes” or “no”). While playing at the Ouija board, two or more participants place their fingers on the planchette, and it moves about the board and spells out words. The Ouija’s actions are generally considered to be due to the ideomotor effect. The EEG results showed that inter-brain activity coherence was observed more dominant in the temporo-parietal junction when the participants shared the goal (in this case, “yes” or “no”) than when they did not. These results suggest that shared ideomotor effects produce brain oscillations in the cortical area, which are used for mentalizing.