A liquid level detection method includes capturing an image of a liquid surface, a structural surface, and graduation markings provided on the structural surface using an image-capturing device to thereby obtain an initial image. Subsequently, the initial image is processed so as to generate a processed image, and a level reference value of the liquid surface is obtained from the processed image. The level reference value represents a height of the liquid surface in terms of inherent characteristics of the processed image. Lastly, a liquid level of the liquid surface is calculated based on a relative proportional relation among the level reference value, an overall height of the processed image in terms of the inherent characteristics of the processed image, and dimensions of any one of the initial and processed images relative to the graduation markings.