Hi there thanks for posting! I apologize for not being able to respond in Japanese. 

Unfortunately it's currently not possible to form new line connections across layers. Lines can maintain connections to shapes across layers if they are moved to the layer after the connection is made but they can't make new connections with shapes on another layer. The one exception to this is that if a line is connected to at least shape on the same layer it will connect to any shapes on the base (page) layer which are moved close to the line. 

​​
But it needs to be connected to at least one shape for this to work. See below- the line on layer 2 is not connected to any shapes so it won't automatically connect to a shape on the page layer. 

 If line connections across layers is a feature you would like to see would you mind adding this to our feature request form? In the meantime as a workaround I recommend moving any unconnected lines to the layers of the shapes you are trying to connect them with establishing the connection and then moving the lines back to their original layer. Please let me know if you have any questions!

日本語の投稿にも対応してくれてありがとうございます。

機能が無いことは残念ですが、正しい情報と回避策を知れてよかったです。

feature request formにも投稿しておきます。