This is "Dorcus curvidens". It's one of the most popular species in Japan. Its size is about 30mm ~ 75mm. It's very hard to find them. Right one is in Japan, but the left one is from Laos.
This is "Dorcus montivagus". It's my favorite species. The size is little smaller than curvidens. They live in high mountains.(about 1000m high) The left one is female.
This is "Lucanus maculifemoratus". The similar species live in many areas in Europe or Asia. They have a short life differently from curvidens.
This is "Figulus binodulosus". In Japan we have a lot of small species. Some of them are hard to be recognized as stag beetles.