This study aims to examine neural correlates of spontaneous deception in a non-competitive interpersonal situation, and the difference in neural correlates between spontaneous deception and instructed deception using functional near-infrared spectroscopy (fNIRS). We used a modified poker game in which participants freely decided whether sending a piece of truthful/deceptive information to other participants. In the instructed session, participants sent truthful/deceptive information per the instructions. In this non-competitive interpersonal situation in the orbitofrontal cortex (OFC) and dorsolateral prefrontal cortex (DLPFC), deception produced higher neural activities than truth-telling. In addition, spontaneous deception exhibited higher neural activities than instructed deception in the frontopolar area, DLPFC, and frontal eye fields. Spontaneous truth-telling produced higher neural activities than instructed truth-telling in frontal eye fields and frontopolar area. This study provides evidence about neural correlates of spontaneous deception during non-competitive interpersonal scenarios and the difference between spontaneous deception and instructed deception.