A state estimation system that is configured to estimate a state of an electrical power system, the state estimation system comprising: a storage section that is configured to store instantaneous value information which is information of instantaneous values received from the electrical power system, and reception measurement information including phase information received from the electrical power system; characterised in that the state estimation system further comprises: a judging section that is configured to judge, on a basis of the instantaneous value information, whether or not effective phase information can be obtained from the instantaneous value information; an instantaneous value processing section that is configured to calculate phase information, and an effective value from the instantaneous value information in a case that it is judged that effective phase information can be obtained from the instantaneous value information, and to calculate an effective value without calculating phase information from the instantaneous value information in a case that it is judged that effective phase information cannot be obtained from the instantaneous value information; and a state estimating section that is configured to estimate the state of the electrical power system on a basis of: the phase information, and the effective value that are calculated by the instantaneous value processing section, or the effective value calculated without calculating the phase information; the reception measurement information; and a topology of the electrical power system. a storage section that is configured to store instantaneous value information which is information of instantaneous values received from the electrical power system, and reception measurement information including phase information received from the electrical power system; characterised in that the state estimation system further comprises: a judging section that is configured to judge, on a basis of the instantaneous value information, whether or not effective phase information can be obtained from the instantaneous value information; an instantaneous value processing section that is configured to calculate phase information, and an effective value from the instantaneous value information in a case that it is judged that effective phase information can be obtained from the instantaneous value information, and to calculate an effective value without calculating phase information from the instantaneous value information in a case that it is judged that effective phase information cannot be obtained from the instantaneous value information; and a state estimating section that is configured to estimate the state of the electrical power system on a basis of: the phase information, and the effective value that are calculated by the instantaneous value processing section, or the effective value calculated without calculating the phase information; the reception measurement information; and a topology of the electrical power system. a judging section that is configured to judge, on a basis of the instantaneous value information, whether or not effective phase information can be obtained from the instantaneous value information; an instantaneous value processing section that is configured to calculate phase information, and an effective value from the instantaneous value information in a case that it is judged that effective phase information can be obtained from the instantaneous value information, and to calculate an effective value without calculating phase information from the instantaneous value information in a case that it is judged that effective phase information cannot be obtained from the instantaneous value information; and a state estimating section that is configured to estimate the state of the electrical power system on a basis of: the phase information, and the effective value that are calculated by the instantaneous value processing section, or the effective value calculated without calculating the phase information; the reception measurement information; and a topology of the electrical power system.