A rotational position sensor comprising: a pair of substantially half-disk shaped first magnetic cores (1, 2) disposed in an opposed relation to each other with a magnetic sensitive device (31, 32) interposed therebetween; and a pair of substantially arc-shaped second magnetic cores (4, 5) disposed in an opposed relation to each other along outer peripheries of said first magnetic cores (1, 2); characterized by further comprising: two permanent magnets (6, 7) fixed in contact with one of said second magnetic cores (4, 5), respectively; and a magnetic plate joined to said permanent magnets (6, 7), wherein said permanent magnets (6, 7) are magnetized in directions opposite to each other substantially in the axial direction of a rotary shaft, wherein magnetic flux generating from said permanent magnets (6, 7) flows successively through one of said second magnetic cores (4, 5), said first magnetic cores (1, 2), and the other of said second magnetic cores (4, 5), followed by returning to said permanent magnets (6, 7) said first magnetic cores (1, 2) or said permanent magnets (6, 7) are rotatable, and said magnetic sensitive device (31, 32) detects an amount of the magnetic flux varying with rotation of said first magnetic cores (1, 2) or said permanent magnets (6, 7). a pair of substantially half-disk shaped first magnetic cores (1, 2) disposed in an opposed relation to each other with a magnetic sensitive device (31, 32) interposed therebetween; and a pair of substantially arc-shaped second magnetic cores (4, 5) disposed in an opposed relation to each other along outer peripheries of said first magnetic cores (1, 2); characterized by further comprising: two permanent magnets (6, 7) fixed in contact with one of said second magnetic cores (4, 5), respectively; and a magnetic plate joined to said permanent magnets (6, 7), wherein said permanent magnets (6, 7) are magnetized in directions opposite to each other substantially in the axial direction of a rotary shaft, wherein magnetic flux generating from said permanent magnets (6, 7) flows successively through one of said second magnetic cores (4, 5), said first magnetic cores (1, 2), and the other of said second magnetic cores (4, 5), followed by returning to said permanent magnets (6, 7) said first magnetic cores (1, 2) or said permanent magnets (6, 7) are rotatable, and said magnetic sensitive device (31, 32) detects an amount of the magnetic flux varying with rotation of said first magnetic cores (1, 2) or said permanent magnets (6, 7). two permanent magnets (6, 7) fixed in contact with one of said second magnetic cores (4, 5), respectively; and a magnetic plate joined to said permanent magnets (6, 7), wherein said permanent magnets (6, 7) are magnetized in directions opposite to each other substantially in the axial direction of a rotary shaft, wherein magnetic flux generating from said permanent magnets (6, 7) flows successively through one of said second magnetic cores (4, 5), said first magnetic cores (1, 2), and the other of said second magnetic cores (4, 5), followed by returning to said permanent magnets (6, 7) said first magnetic cores (1, 2) or said permanent magnets (6, 7) are rotatable, and said magnetic sensitive device (31, 32) detects an amount of the magnetic flux varying with rotation of said first magnetic cores (1, 2) or said permanent magnets (6, 7).