An imaging device and a horizontal direction detection method capable of detecting a horizontal angle of a camera with high accuracy in a simple configuration are provided. The imaging device includes an imaging unit configured to obtain image data by photographing a predetermined subject, an image rotation unit configured to cause a display image based on the image data to be rotated on a display plane step by step, a count unit configured to count the number of pixels of a specific color included in the display image in a scanning line direction within the display plane and obtain a count value for each of rotated display images, and a determination unit configured to determine a horizontal direction of a photographing angle of the imaging unit based on the count value for each of the rotated display images.