I've only read a few chapter (perhaps around 10-15), the story goes like this, the main character was summoned by some being (I forgot if it was a god or goddesses or something else) to a white room, where he was given a skill, chosen by a roulette wheel and then sent to another world to finish a contract with the one who summoned him. Then he got sent back to his world upon completing the contract. And its still going in cycle like that (as far as I've read it).
This is Sen no Sukiru o Motsu Otoko.
The manga starts with the main character hiking up a mountain before appearing in an empty room with a door. Upon going through the door he finds a woman under attack by giant spiders. After he kills one and leads the woman away, he reappears in the room which now has the god who sent him there. The god explains that the main character can get skills by being summoned, however he has to fulfill the wishes of the summoner to keep the skills he gains. The manga then continues in a cycle of him being summoned, getting new skills, completing the tasks given to him, and returning home.