A spring unit including a zigzag spring 40A, 40B; a first sliding unit 20A including a first spring securing part 22 securing a first end of the spring, a first spring accommodating part 28A, 28B accommodating the spring; first slide guiding parts 25 on both sides of the first sliding unit; a second sliding unit 30A including a second spring securing part 32 securing a second end of the spring, a second spring accommodating part 36A, 36B accommodating the spring, second slide guiding parts 35 on both sides of the second sliding unit, wherein the first and second slide guiding parts 20A and 30A are engaged to mutually slide, and the spring 40A, 40B is held inside a space formed between the first and second spring accommodating parts 28A, 28B, 36A, and 36B while the first and second slide guiding parts 25 and 35 are engaged.