A paper container having a top portion (4) that is tightly closed by bonding together opposing surfaces of first top sealing panels (20) and opposing surfaces of second top sealing panels (22), and each of side panels (16) is folded along a first side panel folding line (26) and a second side panel folding line (27) to form a flap (35) having a substantially triangular shape. The top portion (4) is formed by folding down the flaps (35) onto right and left side surfaces of a tubular body (3) through mountain-folding along front-side top panel vertical folding lines (17), front-side top panel vertical folding lines (18), and front-side top sealing vertical folding lines (23), and through valley-folding along second top portion horizontal folding lines (15) and back-side top sealing vertical folding lines (24).