In the present invention, a conditional probability distribution function of received signal which is conditioned with a received signal amplitude (a), a pseudo range (p) and a navigation bit (b.sub.m) which are unknown parameters is acquired from a GPS signal sample S2 on the basis of a condition defined by a working hypothesis set in a working hypothesis setting part WH(i) and then the conditional probability distribution function (P(R.sub.m|a, p, b.sub.m)) is marginalized with respect to the pseudo range (p) to acquire a marginal posterior probability distribution function (P(p|R.sub.m, a)) in a step ST11. Finally acquired is a pseudo range dependent function in proportion to the marginal posterior probability distribution function (P(p|R.sub.m, a)). Then, in a step ST12, a value of pseudo range (p) which gives the maximum value of pseudo range dependent function is determined as an estimated pseudo range.