To provide a propulsion auxiliary device A of a surfboard capable of efficiently providing propelling force and favorable for training of a surfing beginner and a surfing auxiliary device.

This propulsion auxiliary device A of the surfboard 1 is constituted by having a fixing part 2 to a back surface 1a of the surfboard 1 and a propulsion auxiliary fin 3 connected to the fixing part 2 free to oscillate and to change to a wave avoiding posture to avoid a head wave and to a wave receiving posture to receive a following wave. Additionally, this surfing auxiliary device of the surfboard 1 is constituted by having the propulsion auxiliary device A of the surfboard 1 installed on a rear end part of the surfboard 1 by the fixing part 2 and a nose cap 7 installed on a head end of the surfboard 1, extending a difference of elevation between itself and a base bottom part 1b of the surfboard 1 on the head end and to prevent sinking of a head end part of the surfboard 1 down under the surface of the water in propelling by using the following wave by the propulsion auxiliary device A of the surfboard 1.