  In quantum theory, physically measurable quantities of a microscopic system
are represented by self-adjoint operators. However, not all of the self-adjoint
operators correspond to measurable quantities. The superselection rule is a
criterion to distinguish measurable quantities. Any measurable quantity must
obey the superselection rules. By contraposition, any quantity which does not
obey the superselection rules cannot be measured. Although some of
superselection rules were proved, the raizon d'\^etre of the superselection
rules has been still obscure. In this paper we deduce the superselection rules
from an assumption on symmetry property of measurement process. We introduce
the notion of covariant indicator, which is a macroscopic observable whose
value indicates the value of a microscopic object observable. We prove that if
an object system has a quantity that is conserved during the measurement
process, other quantities that do not commute with the conserved quantity are
non-measurable by the covariant indicator. Our derivation of superselection
rules is compared with the uncertainty relation under the restriction by a
conservation law. An implication of the color superselection rule for the color
confinement is discussed. It is also argued that spontaneous symmetry breaking
enables a measurement that the superselection rule prohibits.
