This belongs to sebaceous cyst. It is caused by the blockage of sebaceous glands in dermis, which leads to the shedding of cortex and exfoliated epithelium. Because the content is powdery sebum, also known as powder tumor, the lump is small and round, such as hard rubber, and the small black spot in the center is an glandular tube with epithelial obstruction, which adheres to the skin, can not be lifted but does not adhere to deep tissue, and can move at will, and is easy to occur in the head, face, back and buttocks with rich sebaceous glands. Sebaceous gland cyst is not a tumor and will not become malignant, but it will cause secondary infection. Treatment should remove the cyst.