Rotational spectral lines of the H2CSi molecule are observed in the millimeter-wave and submillimeter-wave regions. The molecule is produced in a glow-discharge plasma of a gaseous mixture of SiH4 and CO. Rotational constants and centrifugal distortion constants have precisely been determined from observed frequencies of 32 a-type transitions with J = 5-4 to J = 12-11 and K-a=0 to K-a=4. From the observed inertial defect, the vibrational frequency of the CH2 rocking mode is estimated to be as low as 331 cm(-1), which is consistent with the large quartic centrifugal distortion constants, D-JK and d(2). Higher-order centrifugal distortion constants up to the octic terms are necessary to obtain a good fit between the observed and calculated frequencies within experimental uncertainties. The low vibrational frequency and the necessity of the higher-order centrifugal distortion terms indicate a floppy nature of the CH2 rocking mode. A preliminary radioastronomical search for H2CSi in space has been carried out toward a few astronomical sources without success.