This flower, Robai(蝋梅), is also called Winter Sweet. Originally from China, Robai blooms in the winter time. It's a lovely yellow flower with an amazingly sweet aroma. Thus the name Winter Rose (I'm just guessing here...).
Every year we can find this delicate flower at the Robai Garden in Koku Koen, one of the biggest parks in Tokorozawa. The sweet aroma always draws a crowd. You just have to follow you're nose to find the garden!