  Herschel observation has recently revealed that interstellar molecular clouds
consist of many filaments. Polarization observations in optical and infrared
wavelengths indicate that the magnetic field often runs perpendicular to the
filament. In this paper, the magnetohydrostatic configuration of isothermal gas
is studied, in which the thermal pressure and the Lorentz force are balanced
against the self-gravity and the magnetic field is globally perpendicular to
the axis of the filament. The model is controlled by three parameters:
center-to-surface density ratio (\rho_c/\rho_s), plasma \beta of surrounding
interstellar gas (\beta_0) and the radius of the hypothetical parent cloud
normalized by the scale-height (R'_0), although there remains a freedom how the
mass is distributed against the magnetic flux (mass loading). In the case that
R'_0 is small enough, the magnetic field plays a role in confining the gas.
However, the magnetic field generally has an effect in supporting the cloud.
There is a maximum line-mass (mass per unit length) above which the cloud is
not supported against the gravity. Compared with the maximum line-mass of
non-magnetized cloud (2c_s^2/G, where c_s and G represent respectively the
isothermal sound speed and the gravitational constant), that of the magnetized
filament is larger than the non-magnetized one. The maximum line-mass is
numerically obtained as \lambda_max \simeq 0.24 \Phi_cl/G^1/2 + 1.66 c_s^2/G,
where \Phi_cl represents one half of the magnetic flux threading the filament
per unit length. The maximum mass of the filamentary cloud is shown to be
significantly affected by the magnetic field when the magnetic flux per unit
length exceeds \Phi_cl \gtrsim 3 pc \mu G (c_s/190 m s^-1)^2.
