This flower seems to be a double flowered hibiscus syriacus.
Flowers of hibiscus syriacus and Althaea rosea are very similar. When I see only flowers, can't distinguish them.
A balloonflower is a plant widely distributed in East Asia, but in Japan we do not see wild species. A single flower of that looks neater and cleaner than a double flower.
By the time the flowers bloom, the leaves are dead and disappeared. So, they are also sometimes referred to as naked ladies. I wonder if the nutrition is sufficient without photosynthesis.