An in-vehicle mobile-terminal holding device holds a mobile terminal on a steering post turnably supporting a steering wheel. The steering post is provided with a holder support member to the tip of which a holder configured to detachably hold the mobile terminal is attached. The holder support member can be housed in the steering post. The holder support member is formed in an arc shape as a whole. The steering post includes a pivot shaft supporting the root of the holder support member having an arc shape such that the holder support member is pivotable between a use position and a housed position. The steering post houses the holder support member in such a way that the arc shape of the holder support member follows the outer periphery of the steering post having a circular column shape.