Provided are a method for exclusively selecting a duodenal fluid sample having a high possibility of containing pancreatic fluid and favorable sample suitability for subjecting to detection of a pancreatic disease marker by evaluating the quality of the sample prior to detecting the pancreatic disease marker, and a method for detecting a pancreatic disease marker using a duodenal fluid sample selected according to that method. Namely, a method is provided for selecting a duodenal fluid sample for detecting a pancreatic disease marker, comprising: (a1) a step of comparing the color depth of a duodenal fluid sample with a prescribed standard color, and (b1) a step of determining that a duodenal fluid sample is subjected to a test for a pancreatic disease marker if the color depth thereof is equal to or higher than the standard color, but that the duodenal fluid sample is not subjected to a test for a pancreatic disease marker if the color depth thereof is lower than the standard color.