1. An image processing apparatus to detect a bubble region included in an image, comprising: a processor; and a memory storing computer-readable instructions that, when executed by the processor, implement: a bubble candidate region setting unit which sets, from the image, a region as a bubble candidate region, the region having a shape corresponding to a bubble region shape model stored in a storage unit; and a bubble region determination unit which determines, on the basis of information belonging to the bubble candidate region, whether the bubble candidate region is a bubble region; wherein the bubble candidate region setting unit comprises a circular region detecting unit which detects, from the image, a region having a circular or arc-like shape.