It is a Chinese elm (Ulmus parvifolia) tree and it looks like you're keeping it very wet. The top inch of soil at least should be allowed to dry between waterings, though don't be skimpy with water when you do it. Also it needs to be in bright light. However, the most important thing is that it's not a 'tropical' tree like a house plant, and needs to live outdoors for life, though it's also very important to know where you live because there may be issues of winter protection. Can you give the approximate location (in the world, the state, etc.)? And in case you've been dunking it to water, don't... it should only be from above, like rain.

Larke, thank you for the answer. I live in Poland (hardiness zone 6). First, I though it was spider mite. Could it be possible?

Possible, along with lots of other things, but it doesn't look like the problem and you need to address the other things. Your tree should be in mostly fast draining grit along with some organic component, though this is not a good time of year for changing it. You do need to learn how to water properly though, and not to a schedule, or as if it was a house plant that you just water 'because' it's a plant. And it looks like it needs a lot more light all day long (which being outside will also help). It's getting late in the season to acclimate it to outdoors, so you need to start leaving it out all day, and prepare a large container of mulch of some kind - sink it into the mulch up to the edge of the pot and have somewhere (once temperatures are below about 20 F.) to keep it, like an unheated but attached garage, or garden shed, etc. Once the soil freezes hard though, don't water at all. Understand that a lot of formal advice on the net is given for trees already in the proper mix, so watering will be a very different business. Look at bonsai4me.com, and go to ibonsaiclub.com/forumotion for more.