To obtain a nonhalogen-based and nitrogen-based flame retardant without adversely affecting polyamide resins by composing the flame retardant of a compound prepared by reacting a specific triazine compound with phenols and formaldehydes in a specified proportion.

This nitrogen-based flame retardant is obtained by composing the flame retardant of a condensation reactional product consisting essentially of a compound prepared by condensing (A) a traizine compound represented by the formula (R is H, an alkyl, vinyl or the like; R1 and R2 are each H, an aryl, an alkyl or the like) with (B) phenols in a weight ratio of (4:1) to (1:2) of the components A to B and (C) formaldehydes in a molar amount of 0.5-0.9 based on the total number of mol of the components A and B. The flame retardant glass-reinforced polyamide resin composition is prepared by including 100 pts.wt. of a polyamide resin, 5-50 pts.wt. of the nitrogen-based flame retardant, 0-20 pts.wt. of a phosphorus-based flame retardant (the total amount of the flame retardants is ≥20 pts.wt.), 5-100 pts.wt. of glass fibers and 0.01-2.5 pts.wt. of a silane coupling agent having epoxy groups, etc. and alkoxy groups in the molecule.