An image monitoring device includes a hardware processor. An image of the outside of a vehicle is captured by a camera. The hardware processor notifies that dirt adheres to a lens of the camera when a ratio of a smooth region in the image is more than a threshold. In the smooth region, differences in luminance value between pixels are small. The hardware processor does not notify that dirt adheres to the lens of the camera when: a ratio of the smooth region in a sky region in the image is more than a threshold, a ratio of the smooth region in a ground region in the image is more than a threshold, and a difference between an average of luminance values of the smooth region in the sky region and an average of luminance values of the smooth region in the ground region is more than a threshold.