For a primitive form f of weight k for SL 2 (Z), let KS(f) be the Kim-Ramakrishnan-Shahidi lift (K-R-S lift) of f to the space of cusp forms of weight det k+1 ⊗Sym k−2 for Sp 2 (Z). Based on some working hypothesis, we propose a conjecture, which relates the ratio KS(f), KS(f) f, f 3 of the periods (Petersson norms) to the symmetric 6-th L-value L(3k − 2, f, Sym 6) of f. From this, we also propose that a prime ideal dividing the (conjectural) algebraic part L(3k−2, f, Sym 6) of L(3k − 2, f, Sym 6) gives a congruence between the K-R-S lift and non-K-R-S lift, and test this conjecture numerically.