To provide a sitting/leaving aid and a chair which enable keeping of aiding effect for a long time during the sitting or leaving period.

Left and right side braces 30 and 31 are arranged between a front brace 10 of a front leg part 11 and a rear brace 12 of a rear leg part 13 to support a seat plate 32. The rear 42 of the seat plate 32 is made flat and supported freely oscillating on both side braces 30 and 31 through left and right brackets 43 and 44. Both the side braces 30 and 31 have parts bent downward at the respective front end parts and bent surfaces 76 on the top surface 67 thereof.