Several types of the fibre-optic magnetic field sensor by employing the magnetostriction for various purposes, are proposed and also realized. The minimum detectable field strength of the fibre-optic magnetic field sensor using conventional Mach-Zehnder interferometric configuration is improved to be the order of 10 0e, by sensitive metal glass material, by fibre-optic monomode coupler, and by computer data processing. The high sensitivity of this fibre-optic magnetometer will be useful in various applications. A small-sized and simple structure of the fibre-optic magnetostrictive sensor employing Michelson interferometer, instead of Mach-Zehnder interferometer, is also proposed and experimentally verified. This structure can be very small because it uses only one fibre coupler, but it is less sensitive due to the laser noise caused by the optical reflection. These types of the magnetic field sensor are usually used for a.c. magnetic field, because the biasing d.c. field must be applied to obtain the highest sensitivity. The d.c. magnetic field can be measured, however, by rotating the sensor, or applying a.c. biasing magnetic field. This magnetic field sensor for d.c. field with a.c. biasing field is also proposed and discussed. This type of the magnetic field sensor is usually used for measurement of the longitudinal (parallel to the fibre) field. But in practical applications, it is sometimes very important to measure the magnetic field in a narrow gap. A new type of the fibre-optic magnetic field sensor is designed to measure the transverse magnetic field. In this sensor, a transverse strain due to the magnetostriction is transformed into transverse strain through Poisson's ratio in the magnetostrictive material and is measured through the strain of the optical fibre. Many other variations of this fibre-optic magnetic field sensors are proposed. These sensors would be used as a cardiomagnetometer, gyroscope, magnetometer for the residual magnetization in rocks, and also in many other application fields.