Cu5(PO4)3(OH)4 (mineral name; pseudomalachite) is a layered compound. A spin network formed by Cu2+ ions (S=1/2) has very unique structure composed of triangles and pentagons.The spin lattice including pentagons as its unit is very rare and new frustration effect can be observed. We already measured magnetic susceptibility, specific heat and high field magnetization of pseudmalachite using natural mineral samples. A magnetic order is found to occur at about 4 K, but calculated magnetic entropy associated with the magnetic order is far less than the expected value. This result suggests that large amounts of spin fluctuations still remain below the ordered temperature. The purpose of this proposal is to study the magnetic properties of pseudomalachite by measuring muon spin relaxation, which provides significantly sensitive probe for the magnetic ordering and spin dynnamics.