A plastic, lightweight formwork for concrete, which is an integrally molded article of a crosslinked polymer obtained by mixing a monomer solution A of a metathesis polymerizable cycloolefin containing a catalyst component of a metathesis polymerization catalyst system and a monomer solution B of a metathesis polymerizable cycloolefin containing an activator component of a metathesis polymerization catalyst system, injecting the mixture into a mold and subjecting the mixture to polymerization and a crosslinking reaction in the mold. (1) The formwork has a rectangular plate having a front surface and having a longitudinal length of 1,000-4,000 mm, a lateral length which is 1/2-1/10 of the longitudinal length and a thickness of 3-10 mm, and a reverse surface of the plate has a frame surrounding the plate and 3 to 6 crosspiece ribs are longitudinally formed on the reverse surface, the frame surrounding the plate having a thickness of 5 to 20 mm and a height of 40 to 100 mm. (2) The crosspiece ribs integrally and longitudinally formed on the reverse surface of the plate have the total thickness of 1/10-1/30 of the lateral length of the plate and the height equivalent to the height of the surrounding frame and are joined to the surrounding frame at both ends thereof. (3) The formwork has a plurality of small reinforcing ribs on at least part of the reverse surface thereof, and the small ribs satisfies the characteristics (i)-(v).