To monitor the gross primary production(P_g) in Lake Shinji, a simple algorithm of the Pg estimation using chlorophyll-a(Chl.a) derived from satellite remote sensing data and the maximum primary production per Chl.a(P^Bm) derived from the in-situ water temperature (Tw) was proposed. Some assumptions for developing the algorithm were verified using field survey data obtained in the lake, 1995-1998. And the algorithm was applied to images and Tw data of different dates in the Lake. The following results were obtained. 1) The area average SPOT/HRV band ratio after offset removal showed high correlation (R = 0.98) for the area average in-situ Chl.a. 2) The primary production per Chl-a (P^B) almost reached saturation (≒P^Bm) under the irradiance below 400μEin/^m2/s condition. 3) The P^Bm showed relative high correlation(R = 0.71-0.73) for Tw. 4) The estimated P_g using the algorithm changed within the range of about 50-700mgC/m^3/hr in the lake.