A unit structural member 20 for building utilizes an M-shaped channel 1 which includes a center U-shaped channel section 2 and right and left U-shaped channel sections 6, 5 continuously formed to share right and left side plates 4, 3 of the center U-shaped channel section 2. A channel trench 7 of the center U-shaped channel section 2 and respective channel trenches 9, 8 of the right and left U-shaped channel sections 6, 5 are opened in opposite directions. The unit structural member 20 for building is constituted with a pair of longitudinal beams 15 formed of long pieces of material of the M-shaped channel 1 in parallel at an established distance and a number of lateral beams formed of short pieces of material of the M-shaped channel 1 in parallel laterally between the parallely-arranged longitudinal beams 15 at established distances in the longitudinal direction of the longitudinal beams 15.