Using monthly medians of ionosonde observations taken over a period including more than four solar cycles (1958--2003) at Kokubunji (35.71¿N, 139.49¿E), Japan and a linear regression model to eliminate solar and geomagnetic effects, we derive the long-term trends of critical frequencies (foE, foF1, and foF2), the E region height (h'E), and the F2 peak height (hmF2). The results show increasing trends for foE (+0.002 MHz/yr), h'E (+0.189 km/yr), and foF1 (+0.0107 MHz/yr) and decreasing trends for foF2 (-0.0058 MHz/yr at noon and -0.0016 MHz/year at midnight) and hmF2 (-0.398 km/yr at noon and -0.505 km/year at midnight). We have also analyzed seasonal and diurnal trend variations. These trends differed for various times of day and months, even with altering signs (except for hmF2). An interesting phenomenon was observed for the first time; that is, the morphologies of the overall seasonal trend variations of foE and foF1 were opposite each other to some extent, although the trends remained positive. A similar phenomenon was found for the seasonal trend variation of hmF2 between local noon and midnight. In addition, comparison of the results from four regression models indicated that the differences induced by linear and quadratic models are slightly significant for foF2 but not for other parameters. On the basis of the average of foF2 data obtained over 5 hours around noon, we have assessed the performance of each of these regression models. The performance differed depending on the month. The results showed that the quadratic model and the linear model that took saturation into consideration performed better than the linear ones which did not consider saturation. In addition, it was indicated that the effect of geomagnetic activity was not significant in regression models at this station.