Pancreas is a gland. It has both exocrine and endocrine part. The exocrine part of pancreas is the part of Digestive system. Because it produces various digestive enzymes. This enzymes are carried to small intestine by pancreatic duct and helps is digestion. So the pancreas like liver and salivary gland is a digestive gland. Digestive glands are part of digestive system.
The diagram of digestive system, (see pancreas is a part of it) :
Pancreas has an endocrine part too. This part produces hormones (insulin and glucagon). Being a hormone secreting gland the pancreas is a part of endocrine system.
The diagram of Endocrine system showing pancreas as a part of it :
So, the pancreas is a part of two body systems : digestive and endocrine.