A temperature fluctuation suppressing device for a heating medium is provided which is capable of sufficiently suppressing temperature fluctuations of the heating medium at the time of supplying collected solar heat for steam generation. The temperature fluctuation suppressing device includes a heating medium mixer provided on a heating medium supply passage configured to supply a liquid heating medium to a heat exchanging device, the heating medium mixer including: a heating medium passage forming member having plural heating medium passages; an inlet member and an outlet member provided separately from the inlet member, whereby the heating medium continuously flowing into the heating medium passage forming member through the inlet member passes through the plural heating medium passages with time-lags to form respective streams, which are then joined together before flowing out through the outlet member.