An onboard device (3) comprises an image acquisition unit, an object recognition unit, an object determination unit, and a notification unit. The image acquisition unit uses a camera (15) to acquire a pre-boarding image and a post-boarding image. The object recognition unit extracts a region of difference between the pre-boarding image and the post-boarding image, and performs processing to recognize an object in the region of difference, via image recognition. The object determination unit determines whether the object recognized is a notification target, and whether the object is present in the vehicle cabin. The notification unit performs notification if an object that is a notification target is present in the vehicle cabin.