A fiber-reinforced thermoplastic resin filament for a 3D printer is formed by impregnating a thermoplastic resin into a plurality of continuous reinforcing fibers, wherein: an average value S of a roundness parameter s of a cross section is 60% to 100%, the average value S being evaluated by (i) taking a photograph of the cross section perpendicular to the axial direction of the filament, (ii) drawing an inscribed circle and a circumscribed circle of the filament in a cross-sectional image and determining the diameter length of each, (iii) calculating the roundness parameter s defined by equation (1), and (iv) repeating (i)-(iii) at a plurality of locations on the filament and calculating the average value S of the roundness parameter s; and the coefficient of variation of the diameter length of the circumscribed circle is 0% to 10%.